Center text over a tie?

[Matt Boersma]

I'm using lilypond 2.12.2 to transcribe a drum solo piece with several
nine-note snare drum rolls:
nineroll = \drummode { sn4:32^~ sn }

But to help beginning drummers (like me), the original sheet music
puts a small "9" just above the center of the tie.*  In lilypond, it
doesn't have to be a tie--a slur or phrasing slur would also do the
trick visually.

I found a 2004 post about percussion where someone requested this
ability, but nothing else related.  I've tried hand-aligning markup
and also using TextSpanners.  I think the snippet "Center Text Below
Hairpin Dynamics" might show the right approach, but my Scheme skills
aren't yet good enough to modify it to draw text above a tie.

Any ideas?


* (Yes, in a closed or "buzz" roll, the actual number of snare hits
isn't set in stone.  But most of us will get two bounces per wrist
stroke plus a final single stroke, so 9 is still good guidance.)