Enable Repeater Shift in USB mode

I'm using my 6600 to operate via Oscar 100. To do this, I transvert to 432 Mhz, then go through another (13cm) transverter for the uplink, and use a Kuhne downconverter (to 433 MHz) for the 10.490 GHz downlink. This means that my transmit and receive frequencies at the 10m fundamental IF are separated by 1.5MHz - a requirement for Oscar 100 operation is duplex working, to allow the transmit signal to be monitored via the downlink, for example to avoid excessive power in the uplink.

In order to make this work, I use Slice Master to sync two slices with a 1.5MHz offset. This is OK, bit it's a clumsy solution when I could do the same thing much more conveniently if it was possible to turn a "repeater offset" on in USB mode, set to 1.5MHz, which would also tie up only one slice.