LF Plugin to mash Multisite-wide posts into one calendar

The setup: Multisite with several “child sites” where the users on the child sites can post blog posts(!), posts that via FeedWordpress enter the mother site′s flow of posts in the loop.

I am searching for a plugin or a function that allow me to “mash” the posts from the child sites into a calendar widget (when child site users post within a given category).

I want the calendar widget to be “all WordPress” meaning I am not looking for yet another Google Calendar feature.

Any suggestions? =)