I think you will be pleased. I should have thought to search for Segway wheelchairs. I know I had seen old videos of a double amputee on a x2 based chair going in a variety of terrains including sand. I couldn't find it, but I did find these:
If these people can do it sitting down you should be good
Segfree based on x2. Has several interesting videos.
http://www.segfree.co.za/the-segfree.htm
Testing Segfree I2 based in sand. This guy uses my technique of going forward in spurts in soft deep sand.
Jeff