I have the following mods installed:
Battlehorn Castle
Battlehorn Castle COBL
COBL
Skyrim Ingredients for Oblivion (including the COBL option)
Item Interchange

What I am looking for is some help in changing Battlehorn Castle COBL (or making a mod on top of it) that updates the ingredients chest in the Battlehorn Castle grotto to include the new ingredients in Skyrim Ingredients for Oblivion (or better yet some way to update it so it includes every ingredient from every mod ala the COBL ingredients books)

I've never actually played through the Battlehorn Castle quest (keep meaning to get to it eventually) let alone used the Cobl add-on. So I'm not sure exactly what you want to do.

But if you just want to add alchemy ingredients or food from Skyrim Ingredients to a container, it's easy if you have the Construction Set Extender. You'll need to open up Skyrim Ingredients and Battlehorn Castle Cobl (if you want to make a patch, don't select an active file). You just need to add the Leveled Items "skAlchemyRandom" for alchemy ingredients (or "skVendorIngredientsFood" for foods) to whatever container you want it to show up in. Change the quantity from 1 to something you like -- 4 or 5 or 50 or whatever. Keeping in mind for the alchemy list there's a 33% chance nothing will drop for each item in your count. If you want specific items in specific quantities you can add those by hand too, just more time-consuming and not random that way. Save. The result should be a mergeable patch.

If you don't have the CSE, it's more complicated but can still be done. Hope that helps.