Can a 150 tow a banner with only 100 hp?
Yes.
http://articles.chicagotribune.com/1994-07-03/features/9407030141_1_aircraft-goal-posts-flyWith everything open but the tool box, the 150 was a common towplane in Southern California back in the early 1970s. If you saw a 150 taxiing up to the fuel island with no cowling, you could be pretty sure there was a hook under the tail.
You could be-bop a 100-mile loop which included the beaches from Malibu to San Clemente, 50kias all the way, go inland back to the LA Basin for fuel and to replenish the ice chest, snag the new message and do it again, four circuits (six if you started early) a day, all summer long.
Some agencies would want a tow all the way down the coast from above Santa Barbara to a turn just shy of the Mexican border. At least one company (Hi, Bob!) would send the banner ground crew up to the start point (staying at a motel), then the pilot would take off from home and deadhead up to the start point. Once the snag was made, the ground crew would toss the sticks into the back of the truck and drive down to San Diego, then wait an hour or so for the plane to come drop the banner. They would then race back up the highway, while the pilot flew home for fuel and lunch, then started the loop again. About the time he would get back to start point, the banner would be set up again, sometimes three times a day during the middle of summer.