I doubt iWeb is sophisticated enough to do this, but is there a way to pre-write blogs or journal entries and mark them as draft or something so they don't publish to the site until you are ready?
IIRC, you can hide the page from being published with the inspector window. If that doesn't work though, try out RapidWeaver, it is like iWeb but with more capability and it should be able to get the job done for you