Wow. Now I'm wondering whether WoW64 does all that, then - one missed API call reference or data structure and boom goes the 32-bit app.

I actually forgot memory managers were a thing when I was writing the parent comment, and momentarily thought the lack of address space translation would mean all 32-bit processes were stuck in the first 4GB of RAM. AST is indeed more of a thunking issue.

Thanks for the info about Win3emu, that's awesome!