I've been looking at that this morning. It looks like the other viable
option I've forgotten is to use the mappedSuperClass, and have users
copy/paste the additional fields / mapping information into the (now
required) application-space extended bundle.
Acme\ForumBundle\Entity\Forum would require adding the addTag(),
removeTag(), etc. fields, and then the ForumTaggingBundle could supply a
Taggable interface so we can still have some clean code organization. I'm
a little confused on how to properly apply the relationship mapping this
way, but it looks like it'll work.
It would have been handy if there was an extension point in the
ProxyFactory so I can generate entities before proxies are created.