I agree that 100UL is not here yet, however I disagree that it is difficult to make - its a simple as adding a gallon or two of Toluene which is rated around 115-120 octane.
Toluene is 114 octane (R+M)/2 and when blended with unleaded gasoline will raise the octane by a direct amount. Example: A 50-50 mix of 87 unleaded and toluene will result in 100 octane. (87+114)/2 = 100.
Toluene is a wonderful fuel additive, and in fact a wonderful fuel itself, for turbocharged engines that operate with lots of heat. However, in large quantities, it's difficult to atomize and will result in a very poor idle with a fuel injected car, and no possibility of cold idle at all with many carburetor equipped engines. It's also a terrible fuel additive in cold locations due to the above.
Long ago (late 70's to early 80's) I worked for a company that developed engine components for Ford Motorsport (SVO) and raced turbocharged Ford cars on a professional level. We learned about Toluene from the F1 teams operating turbocharged engines. We employed it with good results and were able to make 750 reliable HP from 1.7L engines. Subsequently, I owned turbocharged vehicles and used it.
20% was about the limit in non race use. Attempts to take the 94 octane base component of 100LL Avgas without the lead, and add enough toluene to meet the 100 octane requirements results in an unusable fuel. Attempts have been made to add components that enhance starting and low speed operation, without success.
The F1 teams heated their toluene based fuels to 300 deg F before it was injected. That was the secret to it's use.