wow you really are into hobby servos. if i am not mistaken you could do that in the software
using detach function
this function as to stop giving the servo pulse to continue holding that position.
then when you want to use that servo again use the attach function again
theres another function of servo that is attached. this is just to varify that the port is not use for servo and not other things.
so you might not need the transistor. Hope someone could verify this but i am certain that this might do the trick