I'm trying to do the same of this question (multiple linux installation on same filesystem) but I get this result (root filesystem mounted as read-only) . I'm using debian/sid, have successfully modified the initrd /init script and patched the busybox switch_root to chroot into a subdirectory, the filesystem is ext4. The kernel boot but the filesystem is mounted read-only and any variant of mount -o rw,remount fail.
I just guess (not investigated yet) the mount command is unable to resolve the new root / with the old /mountpoint.
Mounting the filesystem as read-write before chrooting (as the second link suggest) is just a ugly workaround, I would like to know if there's a better way do remount rw properly from within the chroot.