Sort of. I'm developing a mod and what I'm wanting to do is provide a texture in the resource pack. Let's imagine I had an item with which I right clicked a creeper. I'd want the entity's texture to change to the texture I have in the resource pack. Is this possible? If so, got any idea how I'd go about it? What I was trying to get at in the original post was that there would be the default texture for the creeper plus some extra textures that I wanted to switch to during the game.
Edit: the reason I mention the idea of the resource pack is I want this to be a purely server-side mod and as far as I can see, that's the only way to provide the extra texture this whole idea happens to hinge on.