Sharing some content across sites

I want to create a number of sites that are all separate in some ways, and common in others.

Ideally, they should share the media, so I don’t have to host multiple copies of graphics. I presume I can do that using symlinks, provided there is nothing stored in the database.

I want pages and blog entries to be unique for each site.

Most of the theme config (I’m using MySiteMyWay’s Elegance theme) should ideally be identical or shared, so I can make a tweak in one site, and not have to replicate it across the rest manually.

But I want to be able to create certain custom post types and have them shared across all sites. For example, I want to be able to create a bio about someone (in this case, a doctor, or chiropractor, or other health practitioner), and be able to “reference” it in the other sites. That is, when the bio is updated in the master list, the changes show up in all the other sites.

I suppose I could accomplish that through database replication, but that sounds like a formula for disaster. There might be a way to “hook” a custom post type so when you access it, it pulls the ID from the local database, then looks that up in the main database. I don’t want to just alias the table (to use oracle terminology) because I only want a subset of the records in the individual sites. Though I could use a query to simulate that.

But I can’t be the first person who wanted to share information across sites. So there must be a plugin for this. But with 23k+ plugins, and not knowing what to search for, I’m at a loss.

Anybody got a suggestion on how to accomplish what I’m trying to do?