Well, I know it's been awhile; but if anyone's interested the mystery has been solved. A friend came over with one of those lighted borescopes, and we found that whatever type of fuel level sender that was installed in the fuel cell takes a 12V+ lead. There are terminals labeled GND and POS (fitting...) on top of the sender unit, and that's where the mystery wire ends up.

Not too familiar with that type of sender, but somehow the possibility of a short causing a spark INSIDE the cell doesn't seem too keen. Oh well, that ISN'T coming out; so I hope it ends up being safe.