Lets consider the internal bluetooth modules used in a variety of Macs (and doubtless PCs too, but never mind that):
Most of the ones I've bothered to look at show up under the USB device tree in System Profile. Now if one such module were connected to the logic board by a cable (there would be obviously be another cable for the antenna), and that cable had 4 pins on either end, what would be the chances of chopping the logic board off the end of the cable, soldering a USB plug onto it, and it working fine in another Mac?

This mod assumes that the USB plug is wired correctly of course, and that the appropriate .kext file is also copied to the new Mac.
I can see no obvious problems with my theory, should it be this simple?