Sample extension that stores additional attributes for an existing entity

We have often considered trying to remove the support for the "extra" column in the SQL versions of many of our entities - however, we know that a number of products use this. If we are to have any hope of achieving this in the future, then we should at least provide a sample of our preferred way of products doing this - i.e. an extension which would store such attributes outside of our entities, yet still catch all the relevant notifications so that they don't get left with stale data. This will be that sample!