Is there an plans on allowing the stacking of concrete? It is used in quite a lot of recipes, and feels like an intermediate. Plus stone bricks are able to be stacked, so I don't see why not.

Is that something you'd use? Since the input ingredients 'decompress' into a more-full belt when processed, it's not common to 'move' concrete at high volume, like copper wire. Could add it in the experiments mod if you'd use it though!

Concrete doesn't go into a lot of stuff in vanilla. Some mods do use it though.
Thankfully, this mod has a very easy to use interface to add items to the stacked list. You could try making a mini-mod to do it yourself. That's how I got into modding.