One seemingly simple (to simple-minded me) solution would be to quit requiring airplanes that don’t need lead to include it in their fuel. The 100LL product has been forced down the throats of carburetors that don’t need that level of octane performance.
If a selector-valve can be placed in an airplane then one can be placed on a fuel-delivery hose which would introduce the lead/TEL into the fuel AT THE PUMP…. or NOT. Then all the little training and personal airplanes that can operate on UL-AvGas can have it and those who need the full 100 OCT w/TEL fuel can also.