Thanks to Pali finding some old osso-backup source in http://repository.maemo.org/pool/maemo3.2/free/source/, I was able to build a tool that decrypts encrypted osso-backup files without the need to have a working N900 Fremantle install.

The download is at http://www.cncmods.net/files/backupdec.zip and is compiled via backupdec.sh. It will compile in scratchbox if you have libosso-gsf-1-dev, libglib-2.0-dev and libssl-dev installed or it should compile in a normal linux environment if you have normal libgsf, glib and openssl headers installed (you will need to edit the sh file pkg-config options for this)

To run the compiled binary, run backupdec in out password where in is the encrypted zip file from the backup, out is the name of the output unencrypted zip file and password is the password used when the zip file was created.

It can also be used to decrypt zip files on the N900 itself if you want to decrypt the files without actually restoring the backup. (i.e. build an arm binary of this and run it on the phone)

If someone has a specific need for a binary of this built for a specific platform, let me know and I will see what I can do to get a binary.

thank you very much my friend
bless you

The source code here is released under the GPL if you want to re-use it BTW.

Also, this only works if you know the actual password as the encryption is too strong to brute force it.

Thank you! That's what I'd been waiting for.

In case there are any archers (Archlinux users) needing this, I put it up in AUR: https://aur.archlinux.org/packages.php?ID=59275

By the way - some help while running would be useful, one forgets. I mean something like 'backupdec --help'...