I know Tinkers' Complement has a config option to allow seared stone to be melted in the meltry but could there be an option, if Ceramics is also installed, to require this so the smelter can be gated behind the meltry? Perhaps even allow grout to be melted into seared stone in the meltry at a low enough temp so it can be powered by coal.

I have no plans to add such a specific feature, especially not one that changes the "vanilla" Tinkers recipes. The original design was to gate seared bricks themselves behind a later machine and use the melter for molten metals until you can craft that.

For what you are asking, it's pretty trivial to add a couple crafttweaker recipes, melter has full support to block a smeltery recipe or add a melted exclusive recipe.