Yes it is possible and the guide you have linked is OK (it is the same method proposed on other forums some time before).
The only limit which that guide not tells you is that on N900 you can create only max 3 logical partitions.

You can create 4 (or more) logical partitions as shown on that guide, but unfortunately the fourth logical partition is not seen by mmc driver and you must use a dm driver for all partitions with theirs number beyond 7

http://talk.maemo.org/showthread.php?t=67472