The main
difference would be installation/deployment. The idea would be that instead of
the 32-bit and 64-bit runtimes being installed directly in parallel on the base
system, they would instead be installed into effectively a chroot with its own
completely 32-bit runtime.

Which changes how software is built, surely.

Tom's use case is where you simply invoke "gcc -m32" on the base
system and (assuming the relevant 32-bit versions of the libs are
present in /usr/lib) it Just Works.

If the 32-bit headers and libs aren't present on the base system then
you have to change how the software is built.