I dug through the code yesterday for the first time to see if I could possibly do this myself. The current plugin api doesn't have methods for getting data from the mic or line in. I'm pretty sure these functins would have to be added to the api. I might look into it a little more and see what I can fix up, but I can't make any promises since I'm in school, and summer classes can be a b*tch. I play guitar myself, and I'd like make a plugin with a tuner, scale viewer, and tab browser/editor so you can make tabs, or upload them from your computer and view them.
again, no promises.
seems like with all the discussion I've seen somebody should be working on this already. In case I do get anywhere though, do you have any suggestions?
http://forums.rockbox.org/index.php?topic=5319.0