I also suspect you're right, Antti, that the stack size is calculated prior to the bytecode being passed to through the peepholer which would have made the built tuple a value in the const array.

Off the top of my head I don't remember where the stack size calculation is made, but my suspicion is it's in the AT -> bytecode step, which would mean making it work from bytecode would mean re-implementing that calculation to work from the bytecode itself (assuming I'm right).