OK, new source code and DLL are posted:
http://ejsurveying.com/sincpacc3d.htmI went ahead and added in Feature Lines as possible entities to select, in addition to Survey Figures and Polylines. It even seems to work fine on entities that aren't completely closed, at least in the couple that I tried.
I changed the names of the file, class, and methods a bit, to get them all more-consistent. So the command is now called HIDEBND, and lives in a class called SincpacC3D.SurfaceBoundaries.HideBoundaries, which seemed a bit more-descriptive than MULTIHIDE.