Perhaps my need is similar: When a member posts to a sub-space and wants to make an announcement, the system now places the announcement only in the sub-space.
Since members arrive at the main page on log in (and may rarely show up in the sub-space without an invitation), it would be nice to be able to place the announcement on the main page and any number of other sub-space pages.
Further, it would be nice to be able to select member to whom an e-mail notification of an announcement would be sent.
Associating content to more than one space is a feature that we can definitely add. The current functionality is that each space is discrete, and only contains content within that space without overlap.
As tags are used more and more, it could be easy to create a "main_announcement" tag and customize the home page to display all information with that tag.