Auxiliary fuel tank.
I currently have a small firm of fabricators (2 blokes with loads of experience who are just setting up their own business) working on a rear mounted auxiliary tank for the g650x bikes. Because they are a new firm they don't have much work yet and are getting this moving very quickly. If there is a demand, I should be able to get hold of an f800gs on demo from my local bmw dealer so they can attempt to measure it up for a tank as well. After they have finished the GX tank of course.
Keep an eye on the GX thread to see how the tank turns out and let me know if you're interested.
Life is not a rehearsal!