I have a two-part question, would an item frame mod be possible, and keep up the updates on the custom items mod :-)
If you mean an Addon (Native Mod), I think it's possible, it just takes some serious logic. About the custom items, I doubt it because if you're talking about ModPE custom items, they tend to use custom item textures there and it's mod pe.
If an item frame mod is to be made natively, it can only take native items as well. But that's just my take on it.
Also, I am not saying I will work on an item frame mod. I would like too because that would be cool. But it's beyond my scope as for now.