An EU cap coming into force next year only allows bonuses of up to double an employee's salary to be paid as long as shareholders approve. If not, the packages are restricted to the equivalent of 12 months' pay.
Prime Minister David Cameron has announced a £2,000 cap on cash bonuses next year and promised to veto any increase in overall pay at the bank.
But there was no commitment to limit bonuses paid in shares and, because the numbers of investment bankers at the institution have been cut, Labour dismissed as meaningless the effect of his veto.
Treasury Chief Secretary Danny Alexander said in a television interview that no approach had yet been made by RBS.
Mr Alexander said: "I would obviously look at what they have to say, but I would be very sceptical as to whether a case could be made. That's a bank, along with the whole financial system, that has benefited from the taxpayer standing behind it and where the strategy is now RBS has to focus on domestic, retail and business customers."