My two retail Windows 8 copies can be activated both in 32-bit and 64-bit (it even came with both DVDs), and with a little tinkering with a text file in a bootable setup flash drive I can install Windows 8.1 directly. Never tried, though, as I'm not willing to format either machine, specially not to put a 32-bit OS in there.
I wonder, though, would it be possible to "fix" this with a microcode update? Too bad I doubt anyone bothered implementing this instruction in an Athlon 64 mobo, as then this could be replicated elsewhere.