A top box with a back rest is the most viable addition. Corbin does sell a stand alone back rest, but I've heard bad things about their service, and I wasn't impressed talking to them at Laguna Seca.
I installed a Givi E52. It's a pretty nice top box. Not color matched. I think a paint job is in order.
As for arm rests, I don't believe there are any aftermarket arm rests for the GT. I'd buy them if there were.
When a government takes over a people’s economic life it becomes absolute, and when it has become absolute it destroys the arts, the minds, the liberties and the meaning of the people it governs.
- Maxwell Anderson