A couple of boarders I know have hay bale bags (people use them to bring hay to shows, etc) that they use to bring/store hay I for their horses. They like to feed a timothy mix in addition to the native grass hay that is provided by my barn. They buy the bales one at a time at the local feed store and keep the bale in their car trunks in the hay bag. If you want to be sure she is not missing out on any hay by visiting her during her feeding time, maybe you could do the same so she would have hay. Maybe the BO could be convinced to give you a bale at a time to keep if he knows you are going to feed part of it every day to your mare when she would normally be eating with the other horses. If not, just find hay somewhere else. Just an idea and probably cheaper per pound than pellets or cubes.