I think part of the problem is trying to find a “drop-in solution”. It seems pretty clear at this point that such a fuel does not exist, and that the engines requiring high-octane fuel will need to have some modifications done to support an unleaded fuel. And it seems to me that if most of these aircraft owners choose to spend $10-20k instead of $1500 to equip for ADS-B, surely they can afford whatever it would cost to modify their engines for unleaded fuels?
It seems like the goal of a drop-in solution is a case of “perfect is the enemy of good”.