Well, I've ordered the flap servo. I think the DXS txm will do the job. Will update when I get everything back together after my last crash.
I have a new fuselage and wing set on order.
For anyone else finding this in the future, it is possible to use the DXS TSM with flaps.
I had this same question, went through with ordering the flap servo and servo extension, (SPMSA382 and SPMA3052, respectively) and it works great once plugged into the flight controller. (I believe it's port 8 on the flight controller)
I didn't have to do anything else, the spare 3-position switch controlled the flaps perfectly once it was wired up. There is one caveat to using the DXS TSM for the flaps, and it's specified in the manual:
"The included DXS ... is not recommended for the optional flaps as it lacks the necessary mixing functions. The use of flaps typically requires an elevator compensation mix, which the DXS does not currently support"
In my experience, it works just dandy, you just need to lower the throttle and ride the elevators when the flaps are down to maintain or lose any altitude.