Wouldn't it be possible to switch to unleaded high octane fuel which is good for most engines (like the Swedish stuff) and offer a certified lead additive for those engines that can't? Obviously the issue there would be the highly poisonous character of such an additive.
Most high compression engines could be made work by adding water injection. The only real issue with unleaded fuel is the risk of detonation. Someone would have to come up with such a water injection system and get STCs for all engines.
I have a O-540 with an add-on turbo normalizer and the only fuel it's certified for is 100LL (and its 100 high lead predecessor). I bet it would run just fine on whatever replacement there is.