Wow ... that is kind of an unfortunate catch-22.

Some things you can try:
- uninstall any apps (i.e. maemo-mapper) that you have installed in the hope that you can get enough room to install xterm to be able to access anything above /home/user/MyDocs (the sandbox that the file manager restricts you to :[ ) ... you might even disable the repos you don't need for this as the cache might be cleared for unused repos and might be enough to fit in xterm.

- someone might be nice enough to package you a tiny deb file that only installs a symbolic link from /home/user/MyDocs/rootfs to / ... this will then show up in the file manager.

- reflash the flash image as that will clear the user portion of the flash on the 770 itself.

Just make sure that when you install maemo-mapper the next time, tell the settings to store the maps on /media/mmc1/somedirectory