Don't underestimate the windage force when you're towing - If it's something vaguley wing shaped like a Topper or Laser the lift can become quite phenomenal - even the relative brick shape of an Oppie will generate a fair force.
Someone (Possibly Bramber or West Mersea, I forget whom) does "bolt on" multi deck bars for trailers - could that be a better bet? It would save you drilling holes in your RIB.
I'm not sure I would trust a load spreading fixing per say, If I were going to tie a dinghy to my RIB for towing I'd go for a variation on what my rib has now - A stainless plate on the outside of the hull & through bolt. FOr what you;re doing a 3mm plate wil lmore than suffice (I have 4mm, but they're designed to sling half a ton of rib from!) If you used Allen Pan Head bolts then the outside stays relatively smooth and even ratchet straps will not be a problem (well, apart from snapping your dinghy hull!)