The 525 does have its virtues that could be strengthened. It is robust, simple and durable. With protective skid plate, etc. it would be hard to harm it. It is so light compared to the competition that handling it for miles in deep sand or muddy tracks in Siberia would be much easier. The reliability could be extended with a thicker base gasket, a cush rear hub, an HT oil cooler, etc. You would be limited in what you could carry even with the ProMoto rack and Dirt Bagz mounted to that rack. You could overcome some of the mileage problems with the JR jetting and the 6.6 gallon Acerbis tank. It would be a whole lot more fun than the competition as well. If you are mechanically adept it should do just fine. If they can do the Dakar on them with every mile being pushed to the limit the 525 should be able to do a RTW trip.
The new 690 Adventure ('08 it is rumored) definitely would work since the 640s routinely do that trip.