Hi Garry,
Yes, I had looked at using skips in this fashion, and it does help somewhat.  However, I'm wanting to have the two spanners appear as one spanner, with a marking occurring midway.  So my current hack is to have one textspanner with a whiteout markup above the note in question and placed over the spanner.

{c1\startTextSpan \override TextScript #'X-extent = #'(0 . 0) c1^\markup {\whiteout "some markup"} c1\stopTextSpan}

Obviously I need to tweak the output as well, but for now it works, with minimal effort.

Still interested in the elegant solution, by the way!

Thanks,
Neil