Vertical edges on TextSpanners?

[Daniel Johnson]

I was recently typesetting a Gershwin song with a very short (2-note) ottava
bracket, which needed the octavation text "8va bassa ad lib." This text is too
long for the bracket, so I considered reducing text size and using column
markup; but the Staff.octavation property takes text, not markup, and it doesn't
support font-size.  Then I tried using a TextSpanner, but was unsuccessful in
getting the vertical edge at the end of the spanner.  It seems that these
vertical edges are only supported for grobs that support the
horizontal-bracket-interface.

This is probably a stupid question, but is there any way of forcing the
TextSpanner to implement the horizontal-bracket-interface?  I'm sure this would
involve overriding the stencil, which would be a daunting (if even possible)
task.  Anyone have any ideas?

--Daniel