True banner towing is extremely hard on the aircraft. I can't even imagine any type of "civilian" drone doing that work, especially if you were planning on flying a decent sized "flag".
I've done a lot of banner towing in my early years and I do remember that we had to install a secondary oil cooler on each tow plane to keep the temperatures within limits. Flying around at almost full power and 60 knots was not a fun thing to do after about the 20th time in the same area.
Now I'm thinking something like an Air Force Predator or similar would do just fine but it might have a hard to picking up the flag after takeoff. It's surprising how many pilots can NOT make a banner pick up in front of the FAA examiner when required!
I got to teach it a lot and many guys just refused to do it after their first few attempts, even after having it demonstrated, literally by the numbers!
But I digress and that was a LONG time ago.
AD