A couple of summers ago we were in Cyprus, and they were towing 6/8 man bananas from the A-Frame of a 6.5m RIB (Humber?). They had mounted it at the top of the A-Frame to make sure that the tow line stayed clear of the prop, but they had then run additional lines from the top of the A-Frame to the cleat at the bow of the RIB to support the A-Frame (the only people on the RIB were the crew - no passengers usually).
When stationary the lines weren't hanging loose, but were 'tight' like a guy rope of a tent (i.e. you could move them up and down a bit). Once under way and towing, the lines were rigid - and I was uncomfortable sitting anywhere near them!!
ANYWAY - my point - the forces applied to the top of the A-Frame are very big - hence the movement in the A-Frame and tightening of the ropes that were supporting it. I'd rather use the towing eyes than just tie to my A-Frame - unless the A-Frame is designed for it.
My towing bridle is just a piece of 3-strand nylon line spliced onto carabiners - but I've put some bungee cord through the 3-strand and clip that onto the A-Frame - when not under load the bungee pulls the rope up and away from the leg - and when under load, the bungee is stretchy enough that it lets all the load go to the towing eyes and not up to the A-Frame. I've not used it that much, but when I do, I haven't managed to foul my prop (yet