Doable, but you end up having to generate a unique sequence for every single instruction in the program because for something like 'mul r1,r2,r3' your going to

Doable, but you end up having to generate a unique sequence for every single instruction in the program because for something like 'mul r1,r2,r3' your going to have to move the information about which registers are being multiplied somewhere else, as well as encode the return address, and as you mentioned the register spill/etc is going to be costly or you have to reserve a couple registers for it. There are a lot of problems doing it that way too, particularly if your on a small microcontroller with limited ram.