I think Wilson is just really caught up in trying to save Amber and although he knows what he is asking House to do he doesn't really think about what could happen. I really wish they had revisited that in the show - even with just one line.

(I have this private theory that part of the reason he does the liver thing in Wilson is that he is is remembering what House was willing to do for him and he thinks that if he is a good friend to Tucker he should be prepared to make that kind of sacrifice too.)

I had hallucinations more in mind with this fic but there's no reason they couldn't be ghosts :) If you are interested I did write a short ficlet for comment_fic a while back where they are ghosts - here