today i've found this snippet: http://lsr.dsi.unimi.it/LSR/Item?id=646
The description says that it needs manual beaming to work, but i've
tried using it with 2.15.36 and manual beaming was not necessary!
The only downside i see is that non-kneed tuplets are positioned
wrongly, so one has to turn this funciton on and off all the time. It
also doesn't support TupletBrackets, but these are usually hidden with
kneed beams.
Just yesterday I was updating the score which was the reason for me writing this in the first place!! Probably about a hundred cross-staff tuplets, no brackets, and a manual adjustment needed for each one of them... Ugh. (The default "flying" numbers happen because the number is placed in the opening of the bracket, whether the bracket shows or not.) In 2.12 and 2.14 it was only necessary to call the functions once for the whole file, but here I ended up needing to turn them on and off repeatedly. (Still a lot of agony spared.)
The conditions in the function were designed to leave ordinary beams untouched, and also to ignore tuplets with visible brackets. (In earlier versions of LilyPond, the functions actually would give OK results with ordinary beams.)
I'll have to investigate, but possibly the actual override of 'Y-offset (or calling other functions related to Y) is having new effects. (You can see one effect by adding \override TupletBracket #'bracket-visibility = ##t -- no effort is made to position the number since the bracket is there, but we lose all horizontal and vertical positioning! This wouldn't happen in 2.14.)
-David