The NXR repeaters have programmable pins on both the 15 pin molex connector and the DB-25 connector on the back that could be set up to control an antenna relay.
However, the NXR-700 will allow you to use a single antenna. From the service manual:
3-4. Antenna switch
If a common antenna is used for the transmitter signal
and receiver signal, switch transmitter and receiver signals
by connecting CN19 to the RX Unit.
If different antennas are used for the transmitter and receiver,
it functions as an On/Off switch for the transmitter
I don't have one of these units in front of me right now, so I can't open it up and see how easy it is (or not). I can see the CN19 connection on the drawings, but I don't know what type of connector it is. The description is sufficiently vague.