Well, I was hoping that with my knowledge of the shop file and forge file I would be able to add or replace things offered in the forge (why do we never get offered El tomes?!), but that doesn't appear to be the case. From what I've gathered, the structure seems to be: MDV_<Type> IID_<Name> MIK_<Weapon Type> (example: MDV_IR for Iron, IID_Ironsword, MIK_SW for Sword). To confirm this, I tried adding the wind edge into the forge. I didn't know this at first, so I just put it into the first empty spot. Turns out that was the spot for the slim sword, and I could see "slim" in the forge then got a crash. I saw there's a "thrown" section (which is for the javelin and handaxe), so I tried putting it there thinking I was clever. No dice. I also tried changing Fire to Elfire, and that also resulted in a crash. "Iron Blade is iron" I thought to myself before getting another crash...
So I don't know if the MDV is the problem here (and there aren't any extra ones), or if there's something else looking for it to be the specific item it is. Searching the whole shop file just for say "iron sword" sounds like torture, so I'd rather avoid that. I think I've gone through the thread and did find some helpful info on page 1, but that was for switching the forge with another chapter's forge which doesn't help here. If anyone knows anything about changing the contents of the forge, please let me know.
In the screenshot, "0xAF76" is the IID for the Wind Edge.