Need something like a TextSpan, but one that goes to the end of a measure . . .

[Matthew Probst]

I'm trying to mark up a score that alternates rapidly between notated and
improvised sections.  I had planned to use a TextSpan to mark these, kind of
like this;

Solo ----------------+

Where the + is a downward line, closing it like a bracket.  The solo sections
might start anywhere within a measure, and might end anywhere within a measure.

The problem with my use of TextSpan is that I can't figure out how to get the
line to go to the very end of a measure containing a whole rest.  The line only
goes to the middle of the whole rest and stops.  I guess the same would apply
for any other rest at the end of a measure, over which this construct is used;
if the span ends on a rest at the end of a measure, I'd prefer that the line
would reach until the end of the containing measure.

I've tried looking through the reference and snippet guide for guidance, but I'm
not sure of what terminology to even search for.  I looked into the alternate
endings; might they be appropriate?  If not, what constructs should I be
researching to find out how to do this?