any rhythm that can be expressed in standard notation can be expressed in abc

Not quite. The standard is ambiguous on this, but as far as I know nobody has ever implemented the ability to nest tuplets. If I remember right, there is an example on the cover of the LP of Brian Ferneyhough's String Quartet no. 2 where he has a quintuplet inside a triplet inside a quintuplet. You would have to represent that as a 75-plet in ABC if you wanted any available software to process it. Ferneyhough does that sort of thing a lot.

Maybe Captain Birdseye plays Ferneyhough on his concertina? If so, can we have a YouTube link? There's a YouTube of the solo flute number from "Carceri d'Invenzione" - that should in principle be doable on the concertina.