Facinating thought... A quick look at the Pentium III documentation show that the microcode* can be updated by the BIOS during the POST...
Yes, that is true. It can also be done at runtime through fiddling with MSRs.
Some with expereince in PentiumIII microcode code could, I'm sure recode the PIII to become 68k compatible...
Sorry to bust your dream here, but this will never happen.
a) Only very few instructions in the P6 family is implemented with microcode, mainly things that are slow and seldom used (divisions etc.)
b) If you know Intel P6 microcode, get a highpaid job at Intel at once, they'll love to have you onboard I'm sure. Reverse engineering CPUs is no easy feat

So no, this will not happen. Even the Transmeta Crusoe is known to be geared towards x86 emulation, and Linus T has said that the native assembly is hairer than mr. Stallmans beard.
So no go here.