Yea so what I think you can do is create an item, like "MyModdedFlintAndSteel" in your mod, then basically verbatim copy the flint and steel class from Minecraft, adjust i as you wish.
Then create a public static boolean method called "isNetherPortalBlock" with a Block or BlockState as a parameter. then just create like an array/list/arraylist with all of your blocks, Just iterate though the array/list and return true if the block matches.
Finally just replace the crafting recipe of flint and steel to return your modded flint and steel.
Is that kind of what you are looking for? Sorry if it isn't, I might have a disconnect on something.