I cannot speak from personal experience. But my daughter is in 6th grade at a 6-12 school, where the middle school and the high school are on different schedules. I asked about this because she is accelerated in math, and she will "run out" of middle school math before she starts high school. I am told that the school has a couple of kids in this situation almost every year. The kids generally end up with a schedule where they attend most but not all of the geometry classes with a high school class, and spend a certain amount of time in the library when they don't have classes on either side. No one mentioned any problem with them getting from place to place, but it's just going two buildings over, and of course these kids are older to begin with.
I would push hard on the idea that he can walk across the football field without needing supervision. In most school districts, students who live closer than half a mile or a mile away do not get a school bus, and are expected to walk to school. Ask why it's ok for the schools to expect him to walk a mile to get to school, but once he's there, he can't walk 100 yards. What if you sign a waiver? And if they really have to be "supervising" him during this time, can't someone watch him out the window? How about if he calls you (or the school secretary) on his cell phone and talks to you while he is making the walk?
If none of that works, but there is a 5th grade classroom that's on the right schedule, can he Skype from one building to another for that time? Maybe there's an isolated computer somewhere in the k-4 building that he could go to participate remotely in a 5th grade class.
