Then you are out of luck, I'm afraid. In its factory configuration, Maemo only exports the MyDocs partition (where your music, pictures, videos and documents are).

You can use the Nokia Suite, if you can find an old version (3.8.54, if I remember correctly; do not, I repeat not use any of the newer Microsoft stuff, it is aimed at newer devices and does not work with the N900), but that will only go as far as backing up your applications and contacts, not a full ROM image backup.

BackupMenu really is your best option. It can create a proper full backup (hence the name). But it requires installing something on the device.