So, IF you were up to it & had the inclination, we reckon the 5-wire QD steering servo could be made to work with a proper Rx if you routed it through a spare servo board. In summary, it would look like "QD servo >> proper servo board >> proper Rx".
If you attempt it, things you'll need to be ready for:
Pull the QD servo apart to see what colour wire is connected to what - you need to know motor +, motor -, pot centre, pot left, pot right.
On your substitution servo board, you'll need to know where those wires need to be connected, so take pics, draw diagrams etc BEFORE you cut the wires from the board.
The substitute board needs to be one that operates a similar size motor to the one in the QD servo. AND ideally the pots in the QD & proper servos need to have similar resistive values (Ohms), measure the whole end-to-end value, the values for centre-to-end for both ends, at rest & at full left/right positions - if they're a lot different to the pot in the substitute servo, it can affect the amount of steering you'll get or possibly confuse the new servo board. You could also consider swapping the pots over if they're physically the same but different values.
It's likely you won't get the pot or motor wiring the right way around first try, so;
If the servo works in reverse (Left-stick gives right-steering) & using the reversing switch on the new transmitter doesn't sort it (Or it hasn't got reversing switches), you need to swap the motor connections AND swap the end connections to the pot.
If the servo turns to full scale & hums or gets warm, you need to swap just the end connections on the pot. If that doesn't sort it, swap the pot wires back & swap the motor wires instead.
Be prepared to spend a whole day messing about with it & have to give up, as with this "Trial & error" method, there's only a 50% chance it'll work & less that it'll work perfectly
(see my first post) 
