(terminology: incipits are short portions of stave(s), one for each voice in the piece, 'hanging' out before the first measure of the piece, indicating what the original has looked like).
Incipit support basically boils down to:
1) (Required) Allowing the initial bar line (and its attached bracket(s), if any) to have an "additional leading space" filled with stave lines but no symbol, as it happens now for notes and rests. It should be possible to apply this to the score first bar line AND to the first bar line after a section break.
This would leave a portion of empty stave(s) at the beginning, where the user could place appropriate symbols to make the incipit up (his responsibility: the program only makes available some staved space).
2) (Optional but useful) Allowing to attach a text to the symbols; it would be useful to add lyrics to the incipit symbols in vocal pieces; of course, there are work-arounds (like linking the text to the first 'real' note of the part), but would raise horizontal alignment issues when the spacing of the music changes.
3) (Optional) Providing additional symbols for white and black squared notation to be used to build up the incipit contents.
Lilypond seems to support incipits, but the documentation on them is lacking (Lilypond doc page ).
If I understand correctly the documentation, Sibelius supports incipits basically as irregular measures (if this is true, it seems to me the 'wrong' way to do it, as incipits should NOT be played or exported as music).