I get the impression in the majority of ditchings of Piper and Cessna light singles, the aeroplane will flip over upside down. Therefore having the banner attached may stop that happening, but what having a banner attached does to your glide may have a significant effect. The drag is high, the glide will be very steep, and I wonder how much elevator authority you will have at slow speed with banner attached to pull out? The banner towing I see in Spain is at very low level with minimal time to do anything.