This would need a proper architecture for cross layer communication first (which is a problem in INET 3) (that's why it was left out from the port). The X layer communication is currently under work, but will not appear until INET 4 which is at least half a year away.
It seems that the radio modules do not add the necessary RSSI values to the controlinfo when they send up the received frames. You can probably either hack in these or fall back indeed to MiXiM.
Probably adding an ISSUE in INET's issue tracker would not hurt either so it will not be forgotten.