I think this is just technically impossible:

I don't know if I'm right:

But Symbian or WinMobile-Phones have a chip on which the OS is stored, which can't be changed by the OS. The so called ROM.
Hard-Resetting such devices means to delete all setting from a second storage - which can be edited.

The n900 is more like a PC in that way, which has a hard disk, and the OS changes "itselfs", manipulates files and the original state could never be restored as it isn't stored anyway.

You could not hardreset a PC either. You can just reinstall it from a CD - like you can reinstall you N900 from your pc (where the os is stored in a file).

Correct me if I'm wrong, but I think that's the way it is.

Bye
MPW