Share wp_posts table across WordPress installs

I’ve been looking online for a way to do this, but haven’t had any luck so far.
Allow me to explain:
We are considering using WordPress for a family of sites (www.klasse.be/leraren, http://www.klasse.be/ouders, http://www.klasse.be/tvklasse and http://www.maks.be)
I’ve got quite a lot of WordPress experience and have done some additional research and it’s all looking rather promising.
But.
Some content needs to be posted on *all* sites, specifically most video content and articles from our print publications.
We currently use a custom-built CMS in which content is entered and the editor can simple check boxes to have it appear on one or more of our sites. In some instances, the editor can add custom descriptions and excerpts per site. All content is centralized in one content table. We’ve found this to be a very powerful and efficient approach to sharing content and tailoring it to different audiences.
We’d like to use the same approach in WordPress, so all posts would come from one wp_posts table, linked to a custom relationship table to determine which posts appear in which site.
Categories, tags, etc. can be specific to each site, but the post content should be shared.
I’ve found lots of advice online on how to cross post articles, but this would create duplicate entries and that doesn’t seem like a good approach for our problem.

An alternative to a shared wp_posts table could be to create a plugin to *access* other wp_posts tables (and their metadata?) and/or wp_query-like functions to combine posts from different content tables into one loop/query. But that, imho, would create issues with duplicate ID’s, so I’m not really sure if that’s feasible.

Anyone got experience with this or something similar, or is this beyond the scope of WordPress?