You need to compile your own bootloader. You might be able to find one for your target in the unsupported builds forum. I don't see a big reason to do this, however. Once you're in the Apple firmware it will remain there until you do a hard reset. Holding Play/Pause in Apple's firmware puts the unit to sleep, it does not shut it down. So, if you mainly use the Apple firmware then just don't do a hard reset and you'll continue to be in that mode.