You could organise the co-op (if I may call it that for now) so that links from a health site were only to other health sites etc as long as all the health sites did not end up with a similar list of links in their blogroll or links sections.
That would likely flag the arrangement to Google as a link swopping scheme or a purchased links scheme - same thing really and bad news.
A better approach would be for links from the content of posts to link to posts on other sites or even to the other's homepage in that case.
Ideally though the links should appear non reciprocal but more random (3 way is easily spotted too!)
In-content links are great links to give and get, particularly from related sites but can also be used for un-related links. That way a health site commenting on where they found great tax advice could legitimately link to the non related site from an in-content link.
Hope that helps
