There were howls of derision last week at Boris Johnson's proposal for a bridge linking Northern Ireland and Scotland. It would lead to terrorists invading Scotland, said critics; hordes of Orange marchers commandeering the crossing with King Billy banners. Civil engineers on Twitter opined that it would be blown down by strong winds, drowned in deep waters, or blown up by military ordnance dumped in Beaufort's Dyke.

My reaction was different. I've no idea whether it is technically feasible – though if China can build much longer bridges across typhoon and earthquake zones, I don't see why a Stranraer-Larne crossing should be impossible, at least in theory. There was more than a whiff of parochial insularity about the way this project was instantly rejected. Och, we're no' China or Scandinavia...

The idea of physically uniting Scotland with Ireland is an inspirational one. We have a similar history and relationship to the UK and the EU. That's why the SNP, while sceptical, have been careful not to rule out the Boris bridge. It could be the catalyst for the creation of that unified “Celtic” fringe, long talked about by nationalists – a counterbalance to the highly centralised UK, based around the London metropolis.

So, it's certainly worth talking about, even if the bridge is a piece of cynical window-dressing by Boris Johnson, which it almost certainly is. He has form here: his previous bridge plans, the London Garden Bridge and his bridge over the English Channel, came to nought.

But he has ordered the civil service to investigate the cost and technical challenges of the Stranraer-Larne crossing. He told school children in London last week that it would cost “only £15bn” – a figure he clearly picked off the top of his tousled head. But the very fact that he put a figure on it should help the case for improving Scotland's infrastructure. That's considerably less than the cost of the London Crossrail project.

Of course, this is a political bridge too far. It is all part of the PM's attempt to unstop the Irish backstop. It is a sop to the Democratic Unionist Party, who are suddenly realising that their Union “lock” on Brexit is being unpicked. Their 10 MPs can no longer dictate terms to Johnson as they did to Theresa May. That's because Johnson's majority is now around minus 44, thanks to his expulsion of rebel Tories. All bets are off.

As this column advised three weeks ago, Johnson has been relentlessly foregrounding the Irish backstop as the key to achieving a deal with Brussels. He is clearly trying to finesse what is called a Northern Ireland-only backstop. This means keeping the North in regulatory alignment with the European Single Market, unless or until the UK strikes a comprehensive trade deal with the EU.

Already, the idea of a common agri-food zone covering the whole of Ireland has been tacitly endorsed by the Irish government and the DUP. This would put the North under EU rules for sanitary and phytosanitary checks. There is already a free movement of people across the whole of Ireland, in the Common Travel Zone, so the idea of a common economic space is not so far-fetched, though tariffs would still have to be collected somewhere

The past reaction of the DUP to any suggestion of a NI-only backstop has been to deliver a resonant No! in the way only Ireland's protestant politicians know how. It would mean a border in the North Sea. They will never accept being cut off from the United Kingdom, and left at the mercy of Dublin and Brussels.

This is where the bridge comes in as a physical manifestation of Northern Ireland's connectedness to the United Kingdom. A few more border checks on the British side of the Great Union Bridge, as it might be called, could be a price worth paying for the DUP, especially if the Northern Ireland Assembly (assuming it reconvenes) has a lock on regulatory divergence with the UK.

As the Times reported last week, there has been a softening of attitudes north and south of the Irish border. There is now talk of the DUP accepting “regulatory divergence” between the North and the rest of the UK, provided that the Northern Ireland government has the final say. “In those situations," the DUP spokesman, Sammy Wilson, said, “we will consider adopting appropriate [EU] legislation if we believe it is to the advantage of industry in Northern Ireland.”

So, no blank cheque on keeping the Six Counties as part of the European Union. But perhaps a pragmatic agreement to diverge from the UK in areas which might benefit the Northern Irish economy. There is more than a hint of a settlement here if the principle of regulatory continuity between the North and Ireland is recognised by Brussels.

Boris Johnson is expected to put Theresa May's Withdrawal Agreement (WA) back to Parliament with a declaration that the Irish backstop no longer exists for practical purposes. Last week's Cummings-inspired political theatre involved Johnson ruling out any possibility of electoral cooperation with the Brexit Party. This was also a warning to Mr Johnson's own far-right “Spartan” Tory MPs that he is not going to accept dictation from them.

He is now under a legal obligation from Parliament to deliver a deal or extension, but this also places an obligation on Parliament to accept any reasonable compromise offered. If MPs refuse to consider the Withdrawal Agreement, as amended by the adapted backstop, the PM will accuse them of creating the circumstances for the very no-deal Brexit that they have outlawed.

Remember, Boris Johnson himself backed Theresa May's WA deal at the third time of asking. A significant number of Labour MPs, like Lisa Nandy, have said that they would now accept the WA to prevent a no-deal. And while the Liberal Democrats have gone full remainiac and insisted on revoking Article 50, that is not the view of the majority of MPs.

Of course, unstopping the backstop doesn't make Brexit itself any more palatable to those of us who think the whole project is an act of national self-harm. But at least we can see where the bridge fits in. If it ever happened it would be a huge financial boost to the North. It would involve improving the roads on both sides and massively increasing traffic in goods, which would benefit the Northern Ireland economy.

As for Scotland, the economic benefits of a bridge would be incalculable. But even if it never happens, it has raised the whole question of why Scotland's infrastructure is so poorly adapted for international trade. It would also strengthen the case for Scotland having a similar regulatory alignment with the EU single market. If Northern Ireland is allowed to remain closer to the EU, why not Scotland?

This is why we shouldn't diss the bridge, even if it is a cynical ploy by the Prime Minister to placate Unionist MPs. The crossing could be Scotland's link, not just to an Ireland that is becoming increasingly unified, but also to the European Union. The Scottish government should be saying: Boris Bridge? Bring it on! But let's see the colour of your money first.