I'm not sure I follow: you first mention Metadata and then data.
In terms of Metadata, you'll be able to store anything you want in the Description field of Account members.
In terms of Data (i.e. annotation, footnotes, etc), as long as the database is not somehow misconfigured, it will be fine to store and display any such text through the interface. If you are going to build custom Connectors that import such data, you might have to pay a bit of attention in your code to handle them correctly using standard .Net practices.