if u had installed dualboot on your N900 togheter with a simple shell environment Linux OS, i might have had some suggestions.

however im too inexperienced to suggest anything in the current situation.

apparantly the nokia flash software can access the rootfs partition so it should be theoretically possible to access the files - if some similar software "out there" can also make a dump of this partition... or if it would be possible to only partially overwrite rootfs.

maybe there is, but not that im aware of