"Text spanner"-ish thing with text in the middle

[James Harkins]

I couldn't find this quickly: for an unmetered section, I'd like to indicate 
duration by tuplet-style brackets, only not a tuplet (no change to note 
durations) and with arbitrary text markup in the middle, e.g. if a tuplet goes

:----- 3 -----:

I'd like

:--------------- 10" ---------------:

TextSpanner seems to assume text at the beginning or end, but not the middle.

Thanks in advance,
James