First, it's not ROM, it's OneNAND. And it's definitely not read-only, as those experiencing low free space on / can attest to.

What you need to do is boot into RescueOS, mount maemo root filesystem, copy / into a partition, unmount the rootfs, mount the home filesystem, copy that into /home on your scratch partition, create a symlink from /home/opt to /opt, remove the pymaemo-optify lines from rcS-late, remove the home-mounting lines from rcS-late, and use one of many ways to boot that root file system, such as Bootmenu (old), a rebuilt kernel with u-boot (new) and pray that it boots.