Note that Connector 13 is now empty. One could drill the rear bulkhead and add another RX Input jack (or add the desired Aux RX connector to the existing arrangement). There's an inherent bonus in not connecting anything to the port: When you press the "RX Ant" switch on the front panel, the antenna is completely disconnected from both receivers.

Notice that the germanium diode (red component visible on the right side of the splitter, attached to the RCA jack) was left in place. There's a reason for this too: We want to clamp the level of RF which could potentially escape that connector if the PIN diodes in the T/R circuit were to fail somehow. (Never mind that the Omni's receiver - by design - is also susceptible to damage from a failure here.)

When I finished the modification I connected a dummy load to the Omni's antenna port and a scope to the new Aux RX port then transmitted a full-power carrier via the Tune function and observed the level on the screen. About 100mV present - less than would be coupled into a receiving antenna from a nearby transmitter.

The really paranoid could place a 40ma, 12v bulb in series with the coax-to-jack connection. I tried a full-power test with a slaved R-71A (with associated Mute function facilitated by an ARB-704) and noticed no degradation of the receiver front end - even with the preamp enabled.