I have done all the backstage tours with an
ECV. You are correct, that you cannot do the Segway as you have to stand and control the machine by your body motion.
Legally you do not have to even give advance notice. However, if you let them know you will be sure to get a lift-equipped vehicle for the tour and they will be ready for you. One time I was on a tour and there were more people with
ECVs than the tour was able to handle, and they had to rush to get a van for the extra people since they did not know about the extras, which inconvenienced everyone else on the tour, including those of us in ECVs who did let them know in advance.
If you want to do the Behind The Seeds tour, because of sharp corners and such it is not accessible for the Disney ECV's but having a standard three-wheel I have never had a problem (and I tend to do that tour at least once a year with out of town firends or relatives).