I would boot something like rescueOS (http://talk.maemo.org/showthread.php?t=75369) from an SD card and recreate the non-ubifs filesystems (I'm not really familiar with ubifs) then copy the original Maemo files to their right places (should also be stored on the SD card).

Edit: actually, you probably don't need to replace anything on the eMMC to get a booting Maemo, just replace everything on the ubifs.

Another edit: actually, since you already have NemoMobile (presumably not using the ubifs at all), you can probably do that from there.

One more: it looks like all you need to do is extract a FIASCO image, take the rootfs.jffs2, and write that to /dev/mtd5