For me a vehicle is a vehicle and and aircraft an aircraft. In my country a vehicle can never be a traditional aircraft - but there are flying cars out there so this is perhaps not the final word about ETBE in California?
For me the logistics with water methanol is no big issue. We have a number of aircraft made in the US that rely on anti-icing and de-icing based on fluid de-icing using ethylene glycol or isopropyl alcohol. I have had these components in gallons in my hangar for a PA-30 and de-icing of propellers. Nothing says that not the same logistics can be done with water-methanol. FBO:s should also not have the problem to assist here if you are en-route.
There are STC:s out there on water methanol injection systems for GA-aircraft for example such as Beechcraft Baron.
Petersen Aviation in Minden NE owns such STC:s I think. Such a STC allows 100 octane aircraft engines to operate on even autofuel with low octane. Typically water methanol injection gives you about 12 aviation octane. With an ETBE AVGAS having 100 octane the additional 12 octane will serve basically the entire fleet that needs more than 100 octane.
There could also be other options – GAMI worked long time ago on their PRISM system, with variable ignition timing. My 91/96 UL AVGAS/ UL94 (which is not an ETBE fuel) was tested by GAMI some 20+ years ago and with their PRISM system on a Lycoming TIO540-A engine. With everything on red in their test-cell the engine did not detonate. It was all observed by among staff from the AOPA and the AOPA-Pilot had an article about it.
I would not be surprised if others engineers with piston aircraft engine manufacturers in the US have worked on this issue. Look at the Lycoming iE2 which states it will work on UL100.
ETBE fuels could also get more than 100 octane but then an efficient scavenger is needed for the additional amount of MMT. MMT is a metal that here works as an octane enhancer. Such efficient scavengers exist outside the US.
In Europe it seems to be we will be allowed to use 100 LL until 2032 so there are another 7-8 years available to refine fuel formulas.
Lead is still the cheapest way to obtain octane numbers - which is appreciated by the GA community.