I did go a little bit deeper in the code for loading/adding the admin extensions. 
When just extending the `StaticContent` has i did it here in the sandbox, to not get an extra admin and with complete copy of such a document like `StaticContent`,  the example won't ever work. The reason is: the check for the instance isn't done by `isinstanceof`, with `$classReflection->implementsInterface($subject)` instead. So as the subject is the `StaticContent` it won't ever be a instance of `SeoAwareInterface` even if i stored a document that extends the `StaticContent`. Means the admin recognizes that the document "yes this is mine", but the admin extension can't be added.

There would be two possible solutions: 
- implementing SeoAwareInterface for the StaticContent in general as it is done with Routes, Menue and Publis-stuff. Has got pros and cons
- creating an basic document with its own admin class

i would prefer the first suggestion, cause all "feature interfaces" are implemented by the `StaticContent`. But as the SeoBundle should not be in the Symfony SE. I don't know if every one wants it. What do you think?