You could do that through another spare channel input to the MCU...

If you have the air pump follow the mixture curve... invert the signal so when the solenoid driver output is high, the air pump driver output is low, and vise versa, you will have modulated tank bias pressure when the solenoid valve is open... Cat mentioned he was using another needle valve to set the bleed off pressure to the tank... so you would trim the tank bias pressure that way... Or if you wanted separate control over the air pump pressure bias curve, you'll have to setup the MCU & sketch with another PWM output, and another channel mix to set the pressure bias curve.