I think technically speaking, a mod could easily add a process for crafting coal (as an item), from a recipe, without it drastically affecting how SeaBlock is played. Like I said, the Omnimatter mods specifically should be compatible (along with some other compatibility mods), forcing the main process to use Omnite (the singular ore from Omnimatter, from which all other ores are crafted in the game, including coal).
But hmmm, having it as a separate mod would be totally possible. It would be much cleaner than making it a setting in this mod, or forcing it upon everyone who happens to have any version of SeaBlock. That being said, I'm not entirely sure which recipe needs to be blacklisted.
I downloaded this mod, and tried recycling the charcoal (looks like a bundle of tiny black spheres) and got the item "crushed coal" (not "coal", specifically). Then recycling the crushed coal, I got "coal". Was charcoal -> crushed coal the recipe that needed to be blacklisted from recycling?
For slag -> crushed stone, I found it, the internal recipe was called "slag-processing-stone", turning slag into crushed stone. I was also able to confirm that adding Omnimatter to the pack adds a second crushed stone recipe (one made directly from stone, in the Ore Crusher from Angel's). 2 equally valid recipes creating the same product, and something that could happen with any number of mods.
I'm actually having a lot of trouble finding whichever mod is adding the charcoal -> crushed coal recipe. Without SeaBlock, but still having all of the other requirements, charcoal recycles into wood pellets, the same way it's crafted. But as soon as Sea Block is enabled, the recipe breaks. Sea Block is quite a mess of a mod, and I can't seem to find the name of the recipe that is causing this. I can't even find it with Factory Planner, which should be able to see "hidden" and "disabled" recipes.