Blitz2, Swdtech2, Morph2, etc...

It looks to me like the code that fills the item menu is at 0x254dc... this code is called before and after battle, and after inventory changes during battle, for example when using Steal. Theoretically you could copy this code, and modify it to skip over any items that aren't in a list stored somewhere else in the ROM. I used a similar approach when I wrote the code to sort Rage names alphabetically... the correct ordering of all 255 rages is stored elsewhere, and that ordering is used to check whether each rage should be added to the menu.

However, this problem is different because you want to keep the old item menu too. And because the item list is quite large, the list is filled only at specific moments like I mentioned before, not every time you open the menu... some in-battle memory is set aside for the item list, so you would need to set aside some additional memory for the new "Potion" list, so that it doesn't conflict with the "Item" list. For this reason, it might be easier to use a command like "MagiTek", because it fills the window every time it is opened, so you wouldn't need to set aside any additional memory.

Either way though, I think that this would be quite an elaborate hack.