Hmm. So is it an adverb or an idiom? I'm going to say idiom, and thus "site unseen" is NOT an idiom, therefore, "sight unseen" is correct. :)
In all honesty, I don't really give a crap, but it's fun to argue once in a while. :D
No, if you haven't sighted the site. It's sight unseen. Or an "unseen site". It's never a "site unseen" - that's bad grammar. In this case it would be written "...[a] site that is unseen".