yes, absolutely a PC processor can be put in the accelerator slot, and it has been done before, the A500 had a 8088 or 8086 accelerator. Effectively giving you a dual processor machine.
It was not used to accelerate Amiga OS.
While this may be possible, its so much cheaper right now just to sit a whole pc down next to your Amiga, and network them together if you want.
Speaking of cheap pc's...I decided I needed a 2nd machine so I could browse while my wife worked....well I picked up a Compaq IA-1 (Flat Screen Color) it is an old internet appliance with 266mhz AMD....well, slapped linux in that machine and now I have an internet capable machine in a cool form factor, the ia-1 was $40, linux free, and I had to add a USB NIC $30, and, I also bought a crossover patch cable and a new keyboard for it.....bringing the total price to still under 100 bucks.....so you see, pc's are mostly free nowadays :-)
btw, people have gotten BeOS, QNX, even Windows 98 to run on the ia-1....(it was originally using only the 'MSN companion software').....
I chose linux because it supported the USB NIC and I didn't have a copy of Windows 98. QNX and BeOS would have been interesting but lacked support for my NIC..