What "old bootloader" are you talking about? You need the Rockbox bootloader. You can uninstall the Rockbox bootloader, but then you won't be able booting Rockbox -- it will only boot Apple OS as you'll be then using the Apple bootloader.
You could try if booting AppleOS from a file works: run ipodpatcher and let it extract the firmware binary:
ipodpatcher --read-firmware apple_os.ipod
Then place the file apple_os.ipod on the root of your player.