I think that's an interesting idea, although I'm not sure how easy or tricky it might be to code in an exception to the automatic turn-skipping for Sebille. It should probably also cost 0 AP.
Just add if(selectedChar == Sebille && Shacklebreak is usable), don't trigger endTurn(), else, endTurn() on the disable check function.
The hard part is making sure everything but Shackle Break is disable since I doubt they had that function prepared. It can probably be done by making it so that Sebille is inflicted with a hidden Athropy, Mute and Cripple on the turn she is acting while Hard'CC, disabling everything but Shackle break.