There's nothing to re-enable. The 64-bit Kernel has only ever been able to load 64-bit drivers. To use 32-bit drivers, you had to be running the 32-bit Kernel.

So to enable these machines to run Mountain Lion, the enterprising dev would have to backport all of the changes from the 64-bit-only Mountain Lion Kernel to the unmaintained 32-bit Lion Kernel.

Right.. Should have though of that. It's the same with other kernels.

Perhaps a simpler workaround would be to run Mountain Lion under VMWare. I believe it supports graphic acceleration and there should be no problem running a 64-bit OS on a 32-bit host with hardware visualization. It might just work.

The simpler workaround is to use the Chameleon bootloader to turn your Mac into what is essentially a Hackintosh. This allows you to boot the 64-bit kernel via the BIOS emulator, bypassing EFI32 entirely.

Catch is it requires two internal drives and a supported graphics card to get it to boot, and the GMA950/X3100 has been cut. This means it's the unavoidable end of the line for MacBooks, iMacs and Mac Minis cut off by this update. It would require someone to write a 64 bit graphics driver/kext which is well beyond the scope of the hobbyist world.

However for us few Mac Pro 1,1/1'2 owners, this means our machines can keep on chugging. EFI is solvable limitation. No support for your GPU isn't. Problem with your solution is any machine old enough to be cut off by this would be pretty crippled by the overhead of running two operating systems at the same time.

While VMware requires a 64-bit CPU to run 64-bit software, every Mac that supports Lion already has a 64-bit CPU, and is capable of running 64-bit software outside the kernel. VMware Fusion 4's OS X video driver is not hardware accelerated, however, so YMMV, especially on older hardware (not sure about the latest Fusion Tech Preview). You're right that it should boot, though.