Hi Brian, normally the best method is instead of using project to make a unfilled curve on your cylinder, use Boolean difference to cut the cylinder with the curve and then you will have all your pieces including your solid filled non-planar piece right from the start.

Here is another discussion thread on this:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1359.2
http://moi3d.com/forum/index.php?webtag=MOI&msg=1359.6


In the future I do want to add a command to fill in a non-planar surface, but even when that is in place it will make a far more complex surface than a trimmed extrusion which is what you normally want for such a surface.

If what is shown in the above thread does not work well for you, please post your .3dm model file so I can take a look at your particular situation.

- Michael