I know this is an old thread, but… I just had the same issue and found
one alternative.
If you are using two cables to connect a set of BasicTX and BasicRX
daughterboards, there is a simple workaround. Just replace the
definition of carrier_sensed(self) to “False” instead of
“self.rxpath.carrier_sensed().” Since your TX and RX paths are
non-interfering, being contained in the cables, you have the equivalent
of full duplex communications and will never sense a carrier to block
your transmit opportunity. I suppose it’s a bit like falling back to a
null mac.
I just did this on a recent svn pull and two Fedora Core 6 boxes, so I
know it works in at least one case. 
This will allow you to show IP
data moving over two USRPs without having to have the RFX-* cards.
Best,
Mike