To get to the point of being able to tow a very heavy sailplane there has to be enough thrust/time to accelerate the sailplane and the tow plane in the field length allowed. If you need a 10,000 ft runway to take off with that combination and don't have that much runway or are operating off a high rolling resistance surface like grass, then that limits the weight that can be towed.
I see that a self-launching sailplane engine has roughly 60HP - so the excess horsepower from above would launch 4 or 5 of those. All up weight of a decent one is around 1800 pounds, so maybe between 7,200 and 9,000 pounds for the group. Not quite an order of magnitude.
I appreciate that having much more margin is better (and the sailplane pilot probably very much agrees) and that there aren't many 10 short-ton gliders, but it's still an interesting comparison to make.
This video for what looks like a smaller banner is interesting - it mentions a 750 pound "fuse" link, so that would limit the drag from the banner. I don't know if a plane with a higher tow capacity would have a higher set fuse. The guy in the video mentions his pick-up is at 150% of stall, but he mentions that most of the flight is as slow as possible to prevent damage to the banner and to give the audience sufficient time to read it.