Due to this thread I found that there was a mistake in my script and I fixed it.
So now it can create lr-mess overlays for gnw from the mame artwork.
I think it's not what you are looking for, though.
It is still an overlay and transparency is always used and not perfect.
I have tried the combination lr-mess with mame artwork but never had any result,
because then the game just won't load.
Perhaps you both can find a way.
It would really be nice if it could work like you want to.