Hi Scott,
One solution is to use a mixer.
Whenever you use a hydraulic function, the mixer is sending a signal to the ESC.
But this solution needs one channel.
Unfortunately I do not have a link how to program the mixer.
Other solution is to use electronic hydraulic switches (eg.
Servonaut BMA or
Brixl Oeldruckschalter - Sorry both sites are in German).
Both systems are between the receiver and the hydraulic servo and can check four (4) servos.
Whenever there is s signal to the servo the module is switching on the hydraulic motor.
At both systems you can adjust a time period the motor is running after the last signal to the servo.
Hope this is what you was looking for.
Cheers,
Markus