Anyone is free to do that (the source to the bootloader is in CVS), but I don't want to have to maintain two sets of bootloaders - I would prefer to spend my time improving Rockbox.
You don't have to use the Rockbox bootloader though - the ipodlinux project is developing a bootloader which is driven by a configuration file and presents the different firmware (Apple firmare, Rockbox and ipodlinux) in a menu that you can choose from along with other features.
It's still in development, but details are here:
http://ipodlinux.org/Loader_2