I am aware of the wording in the manual that Ken quoted. Keep in mind that BRP has similar cautions against all accessories not approved by them...which is any they didn't produce. That includes exhaust systems, air filters, knee pads, luggage, tires, etc. I personally would not fret, as long as your trailer meets the tongue weight and total weight restrictions stated by BRP in their specs. I think this is particularly true if you have the BRP hitch and wiring harness.
There is some question as to how the VSS will respond to another trailer. It is not known yet if the BRP trailer wiring somehow changes the VSS response, similar to what a passenger does. I myself doubt it, since the tongue weight is far less than the luggage capacity, which doesn't change the VSS. There is no real way to know for sure until there is a service manual and a wiring diagram...and even then we will not know what the program actually changes if the trailer wiring is routed through the VCM.
There are lots of unapproved trailers running around out there now on GS Spyders...and also Goldwings, Harleys, BMWs, and so on. I see no reason the RT will not be as adaptable. There will be a need for wiring considerations such as separate turn signals on the trailer, or a 3-2 wiring converter, and also a battery powered wiring isolator, but I think another brand of trailer is entirely feasible if set up right. Your warranty might be in jeopardy if the trailer or wiring causes a failure, though, unlike the BRP unit.