In a face I am currently working on, I have a lookup which I would like to be obligatory (it substitutes alternate versions of dcaron and lcaron depending on whether the following character has a left ascender or certain diacritics -- for various reasons using alternates rather than kerning will simplify my life enormously).

For this particular font, I'd like the user to be able to turn 'calt' off if desired, so 'calt' is not an appropriate place for this lookup to be called.

Would any problems arise if I were to hijack either the 'ccmp' or 'rlig' feature to call this lookup? Clearly these substitutions don't belong in either of these features, but they are the only features I am aware of that most OT-savvy applications call obligatorily. Is there another, more appropriate feature to use? (it would be nice if the OT spec defined some feature which was always active but which had no prescribed function)

André