\once \override TupletNumber.text = #(tuplet-number::non-default-tuplet-fraction-text 12 7)
\tuplet 3/2 { c4. c4. c4. c4. }
\once \override TupletNumber.text = #(tuplet-number::append-note-wrapper (tuplet-number::non-default-tuplet-fraction-text 12 7) "8")
\once \override TupletNumber.text = #(tuplet-number::non-default-tuplet-fraction-text 12 7)
\tuplet 3/2 { c4. c4. c4. c4. }
\once \override TupletNumber.text = #(tuplet-number::append-note-wrapper (tuplet-number::non-default-tuplet-fraction-text 12 7) "8")