With the match and Glasgow's prospects of a place in the RaboDirect PRO12 play-offs slipping away, Van der Merwe had been sent on as a replacement for Tommy Seymour 18 minutes from the end. Soon afterwards, Treviso took an 8-7 lead with a penalty by Kris Burton and seemed to be coasting to their win.
But then, with three minutes left, Van der Merwe took a short pass from Henry Pyrgos just inside the Treviso half, burst past one defender, handed off another and crossed for his score. It was a remarkable try, but more remarkable still as the winger had not played a competitive game for Glasgow all season after suffering a serious shoulder injury on duty with Canada at last year's World Cup. To call it a fairytale finish would underplay the drama of the occasion.
On a more prosaic level, it was also a reminder of what a devastating finisher Van der Merwe could be. Up to that point, most of the plaudits in Glasgow's season had been for their stonewall defence, but his contribution highlighted what they had been missing at the other end of the pitch.
His overall record in PRO12 games now stands at 20 tries from 39 outings, not quite in the Tim Visser league, but a better rate of return than Tommy Bowe, Shane Williams or George North have achieved. With 13 touchdowns (in 24 appearances) for Canada as well, Van der Merwe is the top Test try-scorer playing in Scotland today.
Had things worked out better for Van der Merwe, he might have been breathing down Visser's neck by now. In September, he scored five tries in the space of three games, but things came to a sudden, and almost literal, halt against Zebre when his hamstring gave out. To his credit, the 26-year-old still managed to slip a scoring pass to Graeme Morrison before collapsing on the turf, but the injury put him out of action for the best part of two months.
Small wonder that he is anxious to get back on the scoreboard – and small wonder that he should feel so frustrated after he timed his return to coincide with the point of the season when Glasgow's promising run was just coming to an end. Van der Merwe has not scored a try in his last three outings – but then, neither has any other player in the Glasgow team.
It has been one of Glasgow's strangest features over the years that they have saved some of their best performances for games that mattered not a jot, their celebrated 2009 win in Toulouse being the perfect example. Yet as maddening as that pattern may have been for supporters, Van der Merwe would be perfectly happy to see it repeated in Castres this afternoon, as Glasgow go up against the French side having already lost all hope of reaching the quarter-finals of the Heineken Cup.
"The 1872 Cup is now a big focus for us," said Van der Merwe, looking further ahead to the festive double-header with Edinburgh. "So we can't keep losing games. We've got to start winning and if we can put that right this weekend that will give us a good boost going into the Edinburgh games. We are definitely going to France to get a win and not just show up and play stupid rugby.
"We had two losses in the Rabo at the beginning of the season, but then we had six straight wins and there was a great feeling in the squad. That's something we want to build again and hopefully even improve on sometime in the future.
"It's a great opportunity to go to France. Hopefully the weather is a bit better than here. We are all looking to put in a good performance. For myself, personally, I would like to get back to better form. In the past few weeks our backs have not scored any tries and we want to score some this weekend."
When fit, Van der Merwe has been a shoo-in selection for Glasgow almost from the moment he arrived in 2009. Yet a look through the squad list at the Warriors now suggests that even he cannot afford to rest on any laurels. In the past 12 months, the club have signed Rory and Sean Lamont, Sean Maitland and Niko Matawalu, all of whom put pressure on the wing positions. Does he feel his place is under threat?
"I think the squad has always had depth in terms of the number of players who can play on the wing," he replied. "Obviously we have signed bigger profile players this year. It just gives each guy more motivation to train harder and play harder. I don't look at it as a negative; I think of it as a positive.
"I'll be happy if I can keep the jersey. But if I don't have it one week I will just be training harder the following week. So far it has been good."
With so many players rested for today's game in Castres, a win for Glasgow would probably count as one of their greatest results. It would also give coach Gregor Townsend an enormous dilemma ahead of the first 1872 Cup game, at Scotstoun on Friday. Today's team are clearly not Glasgow's first-choice starting XV, but would he dare to tamper with a winning side?
Of one thing, Van der Merwe is clear. The games against Edinburgh have become the highlight of the calendar for all the players involved. His own background is cosmopolitan – he was born in South Africa and emigrated to Canada in his teens – but he has bought into Scotland's neighbourly rugby squabble with a level of enthusiasm that suggests his formative years were spent entirely within earshot of the M8.
"I get a massive buzz going when I think about these games," he said. "To all the new guys who have come over I've been saying 'just wait for the 1872 Cup'. It's a great pride thing for Glasgow and for Edinburgh as well, something we all look forward to.
"Everyone wants to hit their stride at this time of the year. Everyone wants to be fit and put their hands up for selection."