Having a set tyre in a control tyre class is preferable however I think the reality is that the spread of sizes makes it hard to find a single suitable tyre - like the KU36 - and even if you do there is no guarantee that it will stick around. So the class will probably have to grandfather in a list of tyres.
If your going to have list then you may as well just set up on a parameter like the UTQG. The manufacturers have cottoned on to this being used for vehicle eligibility for a bunch of series - but so what? - if everyone is using - say - a UTQG of 200 - then it will become a defacto standard and somewhat self-leveling.
Personally I like the idea of a UTQG based cut off - I proposed this for Shelbyfest as a cut off determinant but it didn't fly.
Certainly if you compare the UTQG to the 15" parameter for 'Street Class' then the UTQG seems to be a lot more of a level playing field.
Good luck with the quest - if I can get suitable tyres in 15" I might join you
LB