I could see them adding in goal line judges with little effort. Tennis already uses them even before they brought in the tech, so it wouldn't have to slow the game down at all. Just like in tennis, you could have them be subordinate to the main ref, who would make the final call based on their input.

That doesn't sound like too hard a stretch, and you could have the main official focus on the play and misconduct like they seem to prefer to do.