Perhaps doing curves to an adjustment layer and then changing the opacity? Does this sound right?
Custom controls are generally inaccessible to expressions because there is no surefire way to know how their internal data is formatted. It's a just one big black hole and only the people who program those tools know what goes on inside. Your opacity wiggle should work, though. If it doesn't, you could also use e.g. the Blend effect to fade between your original and a duplicate with teh curves applied. Or you do a straight opacity wiggle with two duplicates in teh first palce. Depends on what look you desire, as anturally all of the different methods result in different processing, especialyl in 32bpc mode...
Thank you, the opacity on the adjustment layer worked great. I had a run in with putting curves on an adjustment layer, but found a work around by adding a color balance with null settings first.
I think I understand what you are saying about applying expressions.