This is a real issue, without being able to use these new parameters in formulas it severely limits how useful they are.
I went looking to see if I could fix this myself, and I believe there is a solution. The issue comes from the names of the additional parameters. While the vanilla parameter "items" have names like 'parameter-1' the parameters from this mod have names like 'parameter-a'. The formulas don't seem to understand the latter, but if we modify the names of the additional parameters to match vanilla, e.g. 'parameter-10', the formulas work just fine.