Well, it's a moot point now, as the valve disappeared when I tried getting it out with superglue on the end of a flexible plastic rod -- after I decided there wasn't enough room to manipulate forceps or pliers down there.

I'm still thinking, however, that it shouldn't cause any problems because, AFAIK, the only thing under there is the space between the upper and lower parts of the intake manifold, and they're connected by rubber boots. So it can't get into the manifold, or interfere with any moving engine parts. Can anybody here confirm this?

I guess it could move around in that space, if it does manage to get in there, but, at least, the sides of the nut should prevent it from rolling around. So, hopefully, that's the worst thing that could happen??