I have the same plan. But first, I just want to make a small affordable MIDI synthesizer. The best answers I've found so far (but have not yet tried):
Add a MIDI input interface to your PI. There are many good explanations of this. I liked this one:
Check out: http://www.fluidsynth.org/ which is an open source MIDI/audio ecosystem with dozens of incredible apps. All you would have to do is build their basic software and run it from the command line, specifying one of their sound font files.
If the above works, I plan on using a pair of BlueTooth modules from Microchip. I'll configure them to work as serial port relays at 31.25 kbaud (1 Mhz / 16). The modules run off 3.3V, but most MIDI basically uses 5V, so we take some power from a MIDI OUT, and use it to pump a capacitor and regulate that down to 3.3V to power the BT module. This must be how the Yamaha device works. If you only wanted to go in one direction (from your keyboard to your PI) then you could make a device that only plugged into your keyboard's MIDI OUT and you could power it with a battery or wall wart.
So if that works, I will have a generic BT MIDI "wireless cable". From there, I would explore the BT on the PI and try to use it instead of my device.
And, you probably know, Apple is way ahead of everybody when it comes to BT and MIDI. If anyone knows how to buy one of their evaluation boards without being a seriously company and qualifying for their elite club, please advise.
With time, I will hack and reverse engineer everything Apple does with BT. I will try to abide by all of their rules, even if I can't get a copy of them.