> IIRC, strictly speaking stack machines are not allowed to modify values in place on the stack.

I am curious as to where you heard this. Modifying the stack in place seems like an implementation detail. I mean even if swap wasn't a primitive (which again seems really far fetched), you are still going to have to modify part of the stack in place eventually. It's just in the naiive way you decrement the stack pointer twice then increment it twice, while in the way I outline you don't bother since the length of the stack is the same after the operation is completed.

Probably easier to write less and pseudo code more - I am assuming a downward growing stack here

    # First approach
    temp_a = pop(stack)
    temp_b = pop(stack)
    push(stack, temp_a)
    push(stack, temp_b)

    # second approach
    temp_a = stack[0]
    stack[0] = stack[1]
    stack[1] = temp_a