The idea is good, and I guess you checked the original events of ROP, where there are much less Saxon regiments in Prussian service (and already dispatched to other locations
... which accounted for future 'desertions' in advance...
Now you can then make an event for each unit and test it's desertion later on...perfectly doable. I may suggest you do a template on 1 regiment and see if it works.
Now, as a recomendation, I would not put all the Saxons at Pirna, but have them spread over different Prussian locations and fixed for some time, as in original event...in this way, you make exploits of using them as cheap cannon fodder early on