Andy,
One of the problems with trying to squeeze more functions out of a two function RC outfit is that it can quickly become expensive, more expensive than buying a multi-function outfit!
However, a simple and cheap way to add bow thruster operation would be to place two microswitches such that full rudder movement would operate one or other of the switches. Wired up to give Forward/Stop/Reverse with the bow thruster motor and you have this function. True, you cannot mix the drive motor(s), rudder and bow thruster actions like totally independant functions but if you want to rotate the model when stationary, it works OK. By the way in my experience unless the bow thrusters are very powerful they have limited effect when the model is underway, thus general sailing is not greatly affected. Like most things in life you can achieve 80-90% of perfection very cheaply!
If the motor control system has a sufficently large STOP band then it might be possible to use one position in this band to operate a switch (perhaps via another servo and “Y” lead to turn a water (fire) pump on?
As Barry Foote says, the other functions can just be manually turned On/Off as required.
GlynnGuest