Sounds promising! How about some piezos on the piano to get some signals besides audio? Like vibrations from the body and stuff like that... maybe an envelope follower...

Piezo is a nice idea! Envelope follower could work, too - I'll check that out. Thanks!

you can buy an audio interface for the iphone or (what i did) solder yourself a "breakout"-cable with a TRRS jack. Guess you can buy such a cable somewhere...

Just found an iPhone compatible audio interface in my closet. This will work I hope... ;-)