I'm just a noob developer for N900, so don't know the details...
Open os doesn't mean all aps need to be open. On desktop I'm using nVidia proprietary drivers for my graphic card.
The calendar widget is probably small enough for one man to replicate, and it doesn't integrate anywhere. Calling calendar is simple, but nothing is calling the calendar widget.
Maybe it's more trouble than worth. Sure you can write a whole new app and replicate the functionality. The problem would be the integration. For example when you receive a new message, or tap on send im/sms in contacts. Maybe these could be solved by replacing the original with a new one.
Don't know if there's someone who wants to replicate a whole app just to add some functions which are already implemented separately and work that way.