but when it comes to circular/horseshoe life buoy i run out of options but to sit in the boat with them on my neck!
The silly thing about this rule is that in a RIB you could be back round and pull them out the water in the time it takes to stop, move to the storage location, get it out the bracket and knock them out with it - er I mean throw the horseshoe to them!
Also if someone does fall out at speed you'll probably be beyond throwing distance for one of those things by the time you stop anyway, and if you're already stopped when they fall in the drink, just pull them back in!
As you say, rules for yachts & ferries might not be so appropriate for RIBs - not that this will help you keep it legal. I don't know how big a SOLAS approved ring is, but is there enough room to wedge it between your console and the tube? If not why not buy some of the D- ring patches and some toob goo, then make your own mounting point somewhere on the toobs? That would at least keep it vertical and out the way(ish).