I think this is essentially what I was imagining, although I don’t know if it’d be considered the cleanest possible approach. I guess I just figured that the logic is already structured such that an empty commit is created to avoid having to add special handling for --root all throughout the rebase operation, and so it’s not such a stretch to then automatically consider that commit as always being reachable.
Rebase could also be reworked to not depend on the squash_onto in the case of a --root rebase, but I imagine this would be a much larger change.