I learn something every day.  The file in question was Horizontal Menu - Fixed Width.htm under DNN-Gray.  I edited it and removed the spacer gifs from those two TRs, and after parsing the skin, voila!  The empty columns collapsed.  Didn't have to add the DNNEmptyPane class; the less detailed classes that were already in the skin.css were enough, once we got rid of the spacers.

Now I get to peer at the Minimal Extropy skin and see if I can figure out how to make the NAV fonts smaller - I tried it on my site and the NAV fonts and spacing are so big thatthe menu wrapped around twice, and our logo is big enough vertically that it overlaid the left hand side of the NAV bar (both parts).  Thanks again for your help!  I kind of like Minimal Extropy but there's no way I can use it in that form.