I'm not sure if I fully understood what you're up to.

But if it's ingame... can't you just load the models, copy the bitmap buffers, and when exporting, choose and reference your own bitmap names and save them from your buffers?
It will still have you loading and saving bitmaps, which sounds like extra work at first glance. But it gives you more options for ingame bitmap modification on the other hand. My experience is, that bitmap manipulation/instructions are not THAT slow - unless you need it to happen realtime, which does not appear to be the case.

Of course the export folder should be a different one than the import folder in that case.