If you do that and have only the control signal coming from the F/C which is powered from a completely independent power source, you really should have a common ground between the two battery packs. There is no way for the servo to accurately reference the voltage level of the control signal unless both the servo and F/C have that common ground. You may get away without it for a while but don't be surprised if at some point the servos act funny or fail to respond to stabilization commands from the F/C.
On an MK it really doesn't matter if the servos have a switch or not, the control circuit doesn't become active until the gyro calibration is done (throttle to max and full left rudder). If you're finding this to be an issue it's likely to be because the common ground isn't there.
Ken
P.S. a switch in the power feed anywhere on a multirotor is more likely to fail than a BEC, vibration will kill a switch a lot quicker than the MTBF of solid state components.
hi ken, how's everything been. i do have a common ground in the harness for the camera mount and you're right, the control signal doesn't activate until after the gyro calibration but the signal isn't always clean when it first activates and can cause the mount to spasm before going to it's presets. regarding the switch, the camera mount is the lowest vibration spot on the helicopter if i've done my job correctly and the switch is built into the mount. so far, so good, not too worried about it and boy is it convenient to not have to plug and unplug another battery connection after every flight.
fwiw, i don't think the servos will function without the common ground. my first harness when i was using A123 packs for the mounts was DOA until I added the ground to the signal line.
for anyone that wants to try it, the harness is pretty easy to make using two standard servo y harnesses. signal/ground to the flight control for each servo (clip the red or dark brown power wire), signal/power/ground to the servo, power/ground to the battery (clip the white or tan wire), combine the two power/ground leads from each Y harness to a common plug for the mount's battery. throw a switch in the red wire coming from the battery and you've got an independent, switchable power supply for your camera mount. any switch from an RC kit will work as they do on hundreds of thousands of RC planes/helicopters/cars that have ever been built.
bart