Turns out this gets more convoluted but at the same time, potentially easier to implement:

After a bit more study and experimentation: If the diode in the post above is added to the RF Unit and D26/D44 are removed from the Counter Unit, then the Aux band works perfectly on both its base range and with the "+0.5" switch depressed. We don't need to add D101 or D107 to the Counter Unit as mentioned earlier, either. The matrix lines are properly switched for the required 500KHz shift simply by 9VDC appearing on the 29.5 line, as it will after the RF Unit mod.

Caveat: D26/D44 MUST be reconnected for the 28 and 29MHz band segments to function, and the Aux band WILL NOT function if they are. D26 activates a required PLL Preload bit for all the 10M segments, while D44 has all the 10M switching lines tied to it through a series of other diodes. It ultimately sets the second digit of the display itself (8+ MHz, via BCD logic). This isn't needed for the Aux band in question and if present results in gibberish on the readout.

Which brings us to design of a switching mechanism. (This series of posts is beginning to resemble an electronic scratchpad but the experiments may help others understand how these subsystems of the rig work in concert. I can leave them as-is or edit the series with just the pertinent mod details when finished.)

Logically, what's needed is to omit D26/D44 when the Aux bandswitch line is active. Otherwise, we don't care. Thoughts are to use a pair of PNP small-signal transistors in common-collector configuration. Roughly sketching this out (with mini schematic and pictures to follow): The cathodes of D26 and D44 are lifted from the PCB and each is tied to the emitter of a 2N3906 transistor. The collectors of each transistor go in the now-vacant cathode holes. Each transistor base has the cathode of another 1N914/1N4148 tied to it. Anodes are joined together; a 4.7K, 1/4w resistor connects to them. The other end of this resistor goes to the Aux line at the spot which supplies the cathodes of optional diodes D105-D110. Sleeving is used to prevent unwanted contact between existing circuitry and the new components.

Theory: A PNP conducts from its emitter to collector as long as no base current flows. When base current appears, the device turns OFF. Positioning the transistors at the indicated points in the circuit and using diodes on the bases ensures that no current can flow in the reverse direction, potentially causing problems elsewhere. Desired signal conditions only. During normal 10M operation, current flows through the transistors as normal. When Aux is selected, they're effectively removed from the circuit and no voltage appears at their respective logic gates.

If I see evidence of leakage I may end up adding diodes to the collector side of the devices in a follower configuration - or I may do this right out of the gate to ensure reverse current leakage from Q9 and Q14 cannot occur. All depends on whether or not the gates will reliably change states with a bit more introduced voltage drop.