Sometimes you can, sometimes you can't. Some of the OEM motherboards, HP and Compaq being the worst about it, used modified versions of boards, similar to retail versions but not close enough... they're often configured just slightly differently so that a retail BIOS won't work, and sometimes even using a different BIOS chip that's too small to take a full BIOS (and soldered on so you can't easily change the chip).

But, if you can confirm that a board is otherwise identical to a retail version, then crossflashing can probably be done. I've put retail BIOSes on several Gateway and eMachines boards... in fact, I'm using such a board in my main system.

That's good because I cracked open a machine a while back and stripped it before disposing of the carcass and it had an Asus P5A-B motherboard inside that I've been wanting to do something with. I'm pretty sure it came out of a Compaq or Acer machine.