USRP is a general tool.  Its job is simple: move PCM from Asterisk to an outside app and back again.  It enables most anything we want to support.  AMBE is once of them.  The AMBE source could be DStar, DMR, P25/P2, Fusion, etc.  But that means that the AMBE must be extracted from the stream before it can be used.  That is the hard part.