This looks like a very cool project. As long as I've had a 64-bit x64 processor you've been able to use gcc -m32, but that just uses 32-bit compatibility mode (meaning you can't use the extra r registers or any 64-bit instructions). A 32-bit ABI that runs with 64-bit registers and instructions would be great.

I just hope it's all automatic; I started programming in C just as 32-bit DOS compilers became widespread, so I never had to distinguish between near/far pointers, small/big memory models, etc. I'd like to keep it that way.