I've been researching this in the past, and from what I remember, the only times Windows XP checked for the CMPXCHG8B instruction are at the first part of the setup (i.e. after the "Setup is inspecting your computer's configuration" screen) and when attempting to do an upgrade install from an older Windows version.
The last (leaked) Whistler/Windows XP Beta build to not have the CMPXCHG8B instruction check was 2223. Build 2250 is the earliest build to implement the check.
As an experiment, I replaced the SETUPLDR.BIN file on a Windows XP Setup boot floppy image with the one from Whistler Build 2223. The CMPXCHG8B check was gone, but it showed a BSOD after the "Setup is starting Windows" screen (I can't remember the exact STOP error code, I want to say it was 0x7B (INACCESSIBLE_BOOT_DEVICE), but I could be wrong on that). I tested this on Virtual PC 2007 back then, so I don't know if it would do the same thing on a 486 (real or emulated).
If you have a 486 computer and a Pentium Overdrive CPU for it, you're welcome to try to install Windows XP with the Pentium Overdrive CPU installed then replace it with a 486 afterwards to see if it would still boot.