What are the rules? It can't charge itself. It's already good at driving on freeways and I don't think that part will be so hard, except it's questionable whether it can be done without taking over. But in fairness, almost all of the time when I take over, it's not because I have to and the car will crash if I don't.
It's not a big deal to test FSD ahead of time near supercharger stations, and see which ones FSD has no problems handling, including the trip to and from a highway. And they can add or delete stops on the route without saying that they did the driving. Likewise, they can look at data from FSD beta to assure that people have driven on each part of the route successfully and regularly without taking over. Assuming that they are already looking at the data, they might have a good idea how feasible this is.
As far as FSD goes, I can't get it to go a few blocks in my neighborhood without problems, but that wouldn't be an issue for a coast to coast trip. It might easily avoid complicated intersections, railroad crossings, confusing right turns where it needs to cross a bike path, or any of the things that make it doubtful that they could make many local trips without intervention.
If you remove complex issues on local streets, then whether it's a few hundred miles or coast to coast, it will merely be doing the same highway driving, only longer.