The requirements for the heating system will come from the mortar/thinset you use. The schluter system will have it's own requirements for radiant heating systems as well. It will have a small r-value.

Your well educated tile setter will be able to investigate all the "ins and outs" of an install such as this. Start with the schluter system you want to work with. It will have requirements for mortar/thinset and that will tell you how the heating needs to be dealt with.

We are not planning to use the schluteI realized I was mistaken with the two week wait time for radiant.

What system are you planning on using? Who is doing the work?

I understood you to mean that hydronic radiant heat already exists and you want to add tile. No matter what. The final word comes from the mortar manufacturer.

Clarification on what you're planning would be helpful.