yes that is much clearer.
i am not aware of any piano sounds on the bela, but i guess a sample based approach would work. here are some forum threads that have some (sparse) info, but maybe you could ask the authors of those...
be aware that you won't get the best piano sound like this, but it is certainly an option.
the "random part" playing sounds should be fairly easy to program.
basically take incoming midi messages, according to a probability either pass the midi message unchanged to the soundfont player or do something funny with it (play a strange sound, detune the note to be played, play nothing)
the whole thing you describe could be done on a pc/mac with a great piano plugin (pianoteq for example) and a software (puredata for example) that takes care of the midi routing/randomness. how do you plan to record the EEG? separate from the audio/midi? or do you want all of it to be on BELA?