I don t have a public example to point to (sorry), but I can confirm that it is possible to create Python plugins for an App Engine project. I completed a project a few months ago that does something like this. The crux of the thing comes down to a single line of python:
exec plugincode in someDict
Above plugincode is a string containing some python code to execute, and someDict is a dictionary of globals to execute it in. This is arguably cleaner than using eval(). In our case the globals dictionary contained an instance of an object that the plugincode used to communicate with the system. I can t think of any major limitations with this (or similar) approaches. e.g. plugincode could declare a class, and register an instance of that class as a callback handler etc etc.
In our case we stored the plugin code in the Data Store, and loaded it at appropriate times (e.g. when an instance of the app is started).