Not mentioning the material template system is an omission in the 1.1 specification, I think. I will report it to the thermo SIG as an item to discuss.
I think it is ok for the material template system to return a material template that creates 1.1 material objects.
However, this works only in case you support only either 1.0 or 1.1, or your support for 1.0 and 1.1 are on the same object, as there is no means to tell the material template system which thermodynamic version it should adhere to.