If someone is still interested by a path for this, maybe I found something interesting making grep accross the compiled libs :
I'm using 2.2.4 on a Veer :
in /usr/lib/libPmBtBsaif.so which is a binary file, you find some ASCII sequence with something about bitpool and samplefreq and "%d" in the sting so that a specific integer is inserted (the bitppol or the sample freq returned by a specific function). As it is compiled, there's nothing we can do about these function, but we may try overwriting %d and hardcode the desired bitpool/freq.
I had interesting results with my Sony BT receiver patching the file in hex mode (very important to avoid inserting byte in the bin file) with np++ for instance. I tried to force bitpool 80 and samplefreq 48000, see attached pic. With that, I get a decent quality (without patching, the quality is really really poor), but it may be finetuned, and it may depend on receiver capabilites.
Someone else want to try that ?
As dkirker explains just below, these are just debugging outputs and therefore changing them does not affect audio quality unfortunately. I guess the quality difference I noticed what just due to the fact that I reset the phone right before testing the postmod whereas the phone had been running for a while when I tested the premod. I made some more tests yesterday in the same conditions (after a reset) and the quality is the same with or without the patch.
Sorry for that !