When multiplexing two different REFCLKs, can I multiplex the two IBUFG outputs and use a single DCM for the USRCLKs (the REFCLK inputs of the RocketIO block would still be fed directly from the IBUFG), rather than multiplexing the outputs of two DCMs?
No. To ensure proper clock skew compensation, the use of two DCMs is required.
An alternative could be to do away with the DCM altogether, as described in the RocketIO Transceiver User Guide:
Chapter 2: Digital Design Considerations -> Clocking -> Multiplexed Clocking Scheme without DCM