Is there some amount of medical knowledge that can supplement skiing ability? I saw a lady skiing with patrollers wearing a patrol jacket last month that was at best a level 7 skier. There's no way she could have managed a sled even with nobody on it. I just assumed she must have been an MD or full fledged EMT.
All that said, I don't see any reason why a level 7 skier couldn't safely handle a loaded sled on most types of terrain. Running a loaded sled is, in some ways, easier than free skiing, because the fundamental goal is safe, expeditious transport, not things like carving ability or rhythm. If the level 7 skier has a good, strong snowplow, solid transitions, and a competent falling leaf technique, there shouldn't be too much difference between a level 7 and a level 9 skier in the horns on most trails.
I say "most trails" because terrain obviously matters, and it would obviously not make sense to have a level 7 skier running a loaded sled down you steepest, iciest mogul trail if a stronger skier is available. But could a properly trained level 7 skier get the job done safely and efficiently, if perhaps not as gracefully as a level 9 skier would? I bet they could. They might not do it with the kind of form and confidence you'd need to pass a Senior S&T evaluation, but they could do it.