This is doable for 2 of RCS-4 positions (+12 and AC) after adding a diode and a capacitor. But there may be current limitations.
Thanks Ignacy for giving me a hint on this.
Actually you could do this in first three positions if your tuner will operate at a little less than 12V. If you were to build your power tap using a full wave bridge it would always provide +12V DC. A full wave bridge would out put +12V no matter what you put in +12V or -12V or 12VAC . The output will be a little less due to the voltage drop of the diodes in the bridge.
Position 4 of the RCS-4 outputs NO voltage so that antenna would have to be resonant on the chosen frequency as the tuner would be disabled. You would have to have the tap first - then the tuner - then the switching box.
Clint - W5CPT -