Can you safely use a cast-iron grill pan as a heat diffuser?
I see absolutely no reason not to do that. As you say, the very nature of diffusers is that they aren't smooth. Keep in mind that it's not just extreme heat that you want to avoid, but also extreme changes in temperature. So you want to heat the tagine with the implement that you are using as a diffuser.

Also, unglazed earthenware tagines are usually seasoned. Is your tagine glazed? Apparently that process can also be called "healing" and involve garlic, although the term is from a different culture. Is it necessary to &quot;heal&quot; clay pots?