I don't have a link for it, but I remember seeing a Youtube video of the modchip where the guy used Y and B for GBA games. I might have been mistaken though. If I could find that link again, I'd show it to you.
EDIT: Found it. I was thinking of the DS Chip Advanced, but upon seeing the video again I see you were right. It only used X/Y for turbo A/B. Even the chip's updated version cannot let you use Y/B in place of B/A. My bad, I was wrong.