Simon...
I think you are missing the point a little
The "engine", be it (I)O-320/260, will run quite happily on Mogas,
provided said Mogas gets to the engine, and round the pipework, in liquid form. And some engine manufacturers are quite happy to say as much, others dragging their heels a bit
The problem is the airframe / cowling / fuel delivery installation. And RVs can suffer more by being tightly cowled.
Our RV-8, not helped by a black topped cowling, suffers from fuel vapourisation on the ground, on hot days / hot engine with Avgas. There are ways we could "mitigate" this (mech fuel pump cooling, fuel return lines, run booster pump).
The people to ask will be those regulating your aircraft when it will be flying. They will say whether they will allow it or not. Provided they allow it, or you decide to use it anyway (

), I am sure you'll be OK, subject to some precautions as above.
The UK is one of the stricter places, and following the
Safety Sense Leaflet and
CAP 747, there are type / engine combinations, including O-320 [not sure about IO?] approved.
I do not see RVs getting added to that list, since the fuel system is too variable between builders to get an "approval".
Andy & Ellie Hill
RV-8 G-HILZ
RV8tors