All of my helper functions for overlaying stencils are really just
hacks; they might work for some grobs and fail for others. And when
things stop working, you just have to look for another hack. For
instance, if messing with the stencil for TupletBracket is not working,
perhaps you could use another grob to host the debug annotation.
TupletNumber might be more accepting of modification, which you should
be able to get via (ly:grob-object bracket-grob 'number). But be aware
that you might have to do things the other way around, working from
TupletNumber.after-line-breaking which would mean getting the bracket
from the number: (ly:grob-object number-grob 'bracket).
Your work is great. Finally all these properties are *clarified*, and they are very far from the obscure/missing/messed up documentation. We will have to talk again about this in the next days.
(I did not think about the TupletNumber workaround. Thanks)
Best,
Paolo