I would assume its not possible because the ps2 bios is kept inside the .elf (unlike the ps1 bios which is actually just a file) and its a stripped down bios, to replace it with a bigger bios would mean rewriting the .elf which we can't do since we dont have the source code, or "possibly" it would mean stripping down your retail bios which then what would be the point. But good luck if you do attempt it.