I installed a small strap to the backrest of our Alpacas. Just a loop around the backrest and clipped to the stern tiedown, sure makes it easier to get in the raft. It was always floping down and would not stay up. Velcro on the back rest would be a good improvement. 
That sounds like a good idea to me. You could even stitch just a tiny string into the top seam of the seat and run that back to the tie…