As so often on the abc users' list there is much discussion followed by long silences. In the (last?) abc 2.0 Draft Standard the end of the document says:

End of draft. Features still missing are, at least: repeated tuplets, octava indications, optional accidentals/slurs, microtonal accidentals, tabulatures, guitar chords diagrams, Gregorian chant, ...

(my bold!).

You can of course make use of the !..! and redefinition features to make your own accidentals and hope that at some time in the future you can replace them with some standard version.

Mick