I was looking at the mounting instructions for the quick release touring sissy bar and the instructions look like they would allow the quick release bar to be fitted to a Roadster. Certainly there doesn't look like there is any reason it wouldn't fit.
Here are the instructions. http://triumphinstructions.com/ProdDocs/A9758067-EN.pdf
The only thing I would have thought would be the mounting bolts for the side rails in the rear mud guard might be in a different position. Other than that it looks like it would fit.
From memory someone has said the width of the rear frame is different (Narrower on the touring)
My Roadster sissy bar is 258mm wide to the inside of the brackets.
If someone could measure the Touring we will all know
Edit:
But we should be measuring the width where the brackets bolt on also
Pretty sure the R3T has a lot narrower subframe .. doubt it would fit without a fair bit of modding. Really don't understand why spending 10 minutes max to bolt on a sissybar is seen as such a problem .. guess it's just the age we live in
Not sure what measurement you are looking for. On my Touring the mounting plates (quick release part) are about 12" apart, the mounts for the sissy bar are about 9" apart. If you want I can pull the seat and bar off and get better measurements.
Here is a pic of the Fehling backrest on my Roadster. It looks better on the bike than the picture gives it credit. I'll see if I can get a better pic posted later. I ordered direct from Fehling in Germany. They are great to deal with and communicate well.
No complaints at all, they have held up well so far. When I received it from the manufacturer there was a slight 'nick' in the edge of the fabric. I contacted them and they immediately shipped a replacement part. 5 mins and it was swapped out. So Fehling are very responsive and stand behind their product in my opinion.