I'm currently working on a project which requires booting Gentoo from a file on an NTFS partition.
We've encountered a problem related to unmounting root fs (we need this to close underlying NTFS partition gracefully). It is impossible to unmount root while init is running so we had to add a new 'exec' command to inittab which replaces init whith another executable (capable of unmounting root). Please, take a look at our patch and consider adding proposed feature to sysvinit.