In practice, I'd have thought it would be tiresome trying to ensure that the composition was correct.
Scenario 1) - you add the additive to your aircraft's fuel tanks. You are billed for 10 gallons of fuel, but realise that you failed to check how much was put into each tank - was it 10 into the left tank, 10 into the right tank, or 5 into each. How much will it matter if you have too much or too little of the additive? Will you have to stir the fuel in the tanks of a high-wing monoplane after it's been delivered, in order to mix the additive properly.
Scenario 2) - You own a fuel bowser and you add the additive to it whenever fuel is delivered. But if you are buying a substantial amount at once, why not simply get it delivered ready-mixed and save the hassle?
My understanding is that all the additives invented so far are pretty toxic, so at the end of the day I'm not sure I'd want to handle them neat in any case.
You could invent a device to optionally inject the additive into the fuel-stream as it came out of the nozzle into the tank, so that the same tanker could supply unleaded or leaded fuel. And if you designed it carefully nobody should need to come into contact with the additive.