I still have access to non-ethanol autogas on base. As to the legalese, the beauty of experimental setups is that there is NO STCs for experimental aircraft, and I know the engine doesn't care about FAA mandates when considering the physics and chemistry of octane ratings and compression ratios.

Going back to the certificated game, I seem to agree with you that the STC makers did do a blanket cursory "low compression/ high compression" crossover of avgas ratings to automotive use equivalent octane ratings (different octane calculations, granted). Petersen goes into great detail to discuss the "evils of ethanol", but they do not challenge or provide test data to support the blanket assertion that these 8.5:1 ratio engines "require" 91 octane. I'm inclined to say that provided a suitable positive pressure fuel system (electric pumps at the tanks and insulated lines firewall forward) 87 octane would provide safe and identical performance at a substantial discount when amortized over years of operation. The experimental category is necessary to even make and support this claim of course.

As to ethanol, I think this is another example of Lyco not giving up the gig. There's nothing cosmic about fitting alcohol resistant gaskets and seals, and even then, the claim that a paltry 10% ethanol by volume "eats the thing up" and makes the economy of that fuel use in aircraft engines a "false economy", greatly exagerated. It's not like we're going to be cruising at FL400 at -40F and that emulsified cintilla of water is going to freeze your lines to a fiery crash...