No. By historical development Subsonic began as a browser application using JW Player to playback the media library. JW Player by default only handles a few types of files natively.
http://www.longtailvideo.com/support/jw ... o-formats/Consequently, Subsonic originally output everything to JW as either flv for video or mp3 for audio. Likewise the mobil apps seem to have just followed along, with the one exception I know of being that the Android app will also play ogg. Don't ask me why, but having ogg playback on the app is great as it is a very efficient codec for maintaining good quality at low bandwidth.
I've not tested the iSub app so I don't know if it will play ogg or anything else besides mp3. You should contact the app developer for advice on that issue. I think the original goal was to minimize bandwidth rather than accomodating lossless as most mobil users are on phones using a cellular connection.
You are welcome to test and see if ogg works on your iSub on one of my sites if you like. Just send me a PM and we can set that up. ogg quality settings can be adjusted and I doubt if most human ears can distinguish the difference between ogg on the very high quality settings and lossless. I have a flac rip of Handel's Messiah that I listen to transcoded to ogg on a medium setting and it sounds great. My son is a classicaly trained concert violinist and says the ogg transcoding is remarkable. That's pretty high praise from him.
HTH