Simple question, I'd say. Since a standalone library is just like any other addon, the answer is clear for those. Embedded libraries don't have a .toc, though, so I'm wondering if there's a way to get this done?
When I wrote OneStorage I made a mechanism inside of it that allows it to have multiple data sources registered with it. It then picks one based on which one has the most current data. The downside of this approach is that if you have an addon with the preferred data source that gets removed you loose the data as well. I've thought of a few data redundancy techniques but so far nothing has been done.