The Bitshark USRP RX (BURX) daughterboard has a 26 MHz TCVCXO on board, and uses one of the DACs on the USRP1 to provide the needed warp voltage. These DACs can be controlled from software land, and there are interfaces in both Python as well as C++ for controlling them. See:
for more details about using these DACs.
Thus, if a user has their USRP1 modified to take an external reference clock, you can wire this 26 MHz oscillator to drive the entire USRP1, and then control the warping from software-land. We use this scheme for processing UMTS signals, where the frequency error in the reference clock of a typical USRP1 presents a significant issue if not corrected.