There are several really good ideas in this thread. Unfortunately, I think it boils down to the fact that every single one of them will cost far more to implement than the amount of $ that Disney loses on the soda refills.
Personally, the mugs are okay souveniers, but not THAT attractive, imho. (I have WL lodge ones...)
When I bought them the sign CLEARLY stated that they were for that trip only (length of stay) and I'll happily abide by that policy. However I personally know several people who were told they were reusable, and I would hate to think of the black eye Disney would give itself if they appeared to be reversing a policy that some believed to have existed. In the long run, those mugs hold only about 8 oz of soda (less with ice), which has to cost Disney about 3.5 cents. The bad PR alone would cost far more than that.
There are just times when I really have to put my faith in karma, and know that somewhere down the road, boorish behaviour has its own reward, so to speak.