Hi Will, your idea on deleting the top face and using Planar should do the trick. Once the underlying surface there is a simple plane, you will get a single n-gon there. One note - in this case you should be able to just select the entire object and then do Planar - when a full object is selected you can still run Planar on it and it will look for unattached edges and put trimmed planes on them.

There is actually some stuff built into revolve that tries to detect these types of revolved end planes and replaces them with simple planes. I just tried it on a few simple revolves and it seemed to be working, the only thing is that your line has to come really quite exactly perpendicular off of the revolve axis. If you can post one of the curves that you revolved that did not get a plane at the end, I could check it out.

- Michael