I remember using some Connectix gadget on a IIcx to make it able to address 32 bits. I spent some time in macsbug figuring out how it worked. It was really interesting. It essentially patched the memory manager early in the boot process then forced it to reinitialize itself.

I'm pretty sure Apple licensed that for some version of System 7 (maybe 7.5.something?) so that they could assume all post-boot 32-bit macs were 32-bit clean.