Seperating posts into seperate DB

We have a current installation of WordPress MU, and we have a requirement to separate posts based on a custom field. This field will limit who can and can’t see the post. The first option would allow everyone to read it. The second option would limit it.

Our idea is this: If the blogger posts with the unlimited option, it gets saved into the current WP db. If they select the limited option, we cannot store it in the current WP db, but in another database (security considerations). In the limited DB, we’d store only the post data, and information to tie link it to the current DB.

We think this is possible via a plugin, but I wanted to get some insight from the community to see if this was a road even worth researching.

Thanks!