So in various sections of a piece I want the pianist to lift the pedal slower than usual, achieving an interesting effects as the harmonics fade. In some areas the pedal lift might be slower or faster, and so it would seem that just writing 'lift slowly' won't be effective. For this reason I'm wandering if there might be a graphic solution for notating pedal lifting, such as using hairpins?
As far as I know there isn't a standard notation for what you want.
I'd probably use a half-pedal marking, but make the rise or dip in the pedal line wider - and be sure to include a performance instruction explaining the marking.
The effect you get by lifting the pedal slowly is similar to what you get when playing flagolettes, or harmonics on string instruments. You could adapt the standard simbol for that, and indicate with a line the duration of it.
You can make a remark at the beginning of the piece (perhaps write a foreword). Make a legend for the individual pedal stages and describe what sound the player should achieve.
*use pedal slightly. Allow strings to vibrate w/harmonics only.
Modern composers are very strict sometimes in giving detailed notes regarding the performing of their(!) musical intentions in the piece. Do the same.
This is a case where linear graphic notation can be useful as it can give precise fine grain visual information that mere words are single symbols cannot....