maybe a silly question, here's the setup:

I have a high end '85 Peugeot. The seatpost is fused with the seat tube. I know this because when trying to remove it, the single nut at the end of the tightening belt fell into the seat tube, where it now rattles and slides back and forth when I flip the bike upside down. I can't remove it because the seatpost and seat tube think that they are the same piece of metal. It is quite annoying.

Now, what I'm wondering is if I can get it out by removing the bottom bracket (which could probably use a cleaning anyway) or if the bottom bracket shell is a solid tube with the other parts welded or lugged onto it. Any ideas?

It depends a bit on the actual construction. Lugged can have a fair sized hole in there. If welded or brazed to the a BB that is pipe shaped, there will normally be a hole to act as a pressure release. This is normally too small to allow a nut to pass. I wonder if there is anything you can do to free the post, like heat the seat tube with boilng water and hope it expands enough to break the bond. Or try feeding some oil in there. If all else fails and you are left with no alternative other than a rattle, you might be able to get a tube with some epoxy through the hole in the bb and leave enough epoxy there that it bonds the nut.