While I don't think it is "allowed", I have seen lots of people going the snack route while on the buses, not just DME. I have low sugar, and I have eaten a protein bar while on the bus.
My friends just did this during Christmas/ December, on departure day, They had breakfast at Chef Mickeys, then headed for DTD, Shopped around for a while, cashed in all the snack credits at Goofy Candy Shop (ask for a box for your treats.) Then went into Earl of Sandwich and used up the rest of their meal entitlements for to go. They did not get any hot sandwiches - So cold sandwiches ( they request mayo and mustard in packages) chips and drinks. The headed back to the resort, on DME and headed for the airport. Their flight was delayed a few hours, so they dug into the to go in the main concourse before going thru security. It worked well for them.