I had the same thoghts regarding "solo riding" sit and allocation of licence plate holder + stop and turns signal.
it can be that mud guard plate from GS ADV can be, after certain amendments, used at 9T as a licence plate holder:
regarding the stop and turns signal - the best potion I see is under the short tail of solo sit or above the license plate.
On the official pictures of solo version, there are no stop and turn lights, it means they can be taken off, it means that their wiring is having a certain plug at the end.
it means that you can buy a spare wiring with plug from the dealer, make it shorter (if you will install the lights under the tail of solo sit) or enlarge its length (if you will install the lights above the licence plate above the rear wheel).
you will still keep the original length wiring, just in case you will want to make the bike back to "rider +1" configuration.
this is in theory.
looking forward to find out what will be in reality.