[As others are finding, this post, originally sent in early this
morning, never made it to the list. I wonder if Dan has had a chance
to look into what the problem seems to be.]
I don't think map makers concern themselves very much with the
question of what to do with contour lines over horizontally level
surfaces for at least two reasons: a) they don't generally exist over
large enough areas to have to deal with that carefully and b) they
never correspond to the altitude of a contour line.
The only problem would arise when dealing with horizontally level
surfaces of nonnegligible extent that are *close* to the altitude of
a contour line. Even then the only problem is trying to figure out
whether the altitude is above or below the contour level.