Garabaldy,
Perfectly possible
First you might like to consider the wonders of FLJ's gadgets - some of which are intended to do EXACTLY what you describe. You may well know this already - and indeed he does like a modest amount of cash for these wonders
So if you are going to do this - you can do exactly what you suggest - probably using microswitches
Assuming that the thruster is a normal DC motor what you need to do is to turn it on one way with say, full left rudder and t'other way with full right.
The weapon of choice is microswitches, and to reverse a motor you need basically two connected as a reversing crossover - I will find the circuit and post it if it would help.
You do not want the thruster running all the time so the switches need to be arranged to (as you say) function only at the ends of the servo travel.
As a say of simplicating the installation remember that you can drive the switches off a second servo plugged into a Y lead on the rudder servo - you can then do all your switchery on the slave servo, and mount it wherever seems convenient, and don't need to fit anything near the rudder or servo.
I will hatch a switch diagram. I won't tell FLJ if you don't
andrew