@Robbiethe1st: you didn't read my subsequent messages, obviously
As you stated in your first post of the backupmenu thread, not all versions of mtd-utils make working images. To prepare a working zlib-compressed image I had to use the own mtd-utils of maemo5 on my n900 device.
That image worked perfectly when flashed, without making the device slower (which I feared), and more than doubled free space.

The only thing which gave me problems was on-device untarring of the tarred rootfs archive - my device stalled and thrashed optfs (where I was expanding the archive). I used my ubuntu box to untar rootfs in a directory under optfs, with my n900 connected through usb in backupmenu (option w).