Ok, in brief:

• Try pairing the HSP and doing sound through the headset. It is expected to fail, either on pairing or playback.
• Add the audio.service file, and comment the if/fi lines qwerty12 quoted in bluez-utils. Try pairing and playing sound again. (For consistency, it may be best to delete the entire configuration (/system/ in gconf) for the paired headset between tests.)
• Alter /system/osso/connectivity/BT/unsupported_hw -- probably removing the RX-48 entry and leaving an empty list is fine, but deleting the key may not be. (And test again.)

For the last test, you should be as near non-RX-48 status as we currently know how to make it, so if it doesn't work... either there's some other setting somewhere, or there's something singular about the hardware.

If, OTOH, it does work at this point, then maybe test for interference with WiMAX; if so, they probably disabled it because they felt that rendered it useless, and we've got a cool hack with niche utility. If it seems unaffected, or no worse than WiFi, then it looks like a political issue, i.e. Sprint just didn't want people using their N810Ws as VoIPphones, and we are t3h liberators! (Not a conclusion I want to jump to prematurely, but a fairly obvious one if testing does turn out that way.)