I am a fan of the extensibility of the CMSes. You can upload some code (usually PHP), authorize it from the CMS admin panel and it's running.
I wonder if it is possible in Google App Engine. I haven't checked the extensibility of existing CMSes for Google App Engine, but if there is any of them that supports plugins I would like to know how they did it, and whether they are JS plugins only, or if they support Python/Java plugins too.
Nick Johnson from Google wrote an entire blog post series on how to write a blog system for app engine. If it doesn't do what you want, I am sure that you can extend it but normally a blogging system is sufficient for a CMS for most people.
Actually I see no conceptual problem with supporting plugins in App Engine application. For example on Java you may fetch plugin jar to memory from data store or memcache (on application initialization phase), and then use custom class loader to load plugin classes as needed). Actually you even may load classes from request data and evaluate them on the fly if needed (how we do it in AppWrench Java console).
Regards, Pavel.