hehe I can see me attempting something like that one day. Fiddly job and of course no pc lasts forever, so you'd have to be prepared to scrap it after 3-5yrs.
As long as the box is big enough and you put flying leads to the connectors rather than butt the motherboard up to the case, there should be no reason why you couldn't upgrade the motherboard every few years and have it for a while.