I fly model aeroplanes from Dunsfold Aerodrome near Cranleigh in Surrey. For some years there was a chap who did occasional banner towing in a Wilga. The technique was to take off, double back, and fly low with a hook dangling on a line to catch another line strung between two poles. If the banner was caught, which it wasn't always, the pilot then pushed the throttle to the stop and went off as near vertical as he seemed to be able to achieve. Some of the antics involved in doing this looked to me to be close to the limits of physics and the airframe. Though I'm neither a pilot nor an engineer, at least, not of full-size aeroplanes.
The fun all stopped a year or two back when the pilot noticed creaking coming from above his head in flight. Investigation on landing showed that the mainspar was not entirely in one piece. The plane then disappeared, presumably back to Poland for refettling. Mind you a similar plane has recently been seen at the airfield and it is blue, so perhaps the one involved here.
Why can't you just take off with the banner stowed somewhere, and deploy the thing at height? It would at least give you a chance to deal with SNAFUs at altitude.