Plugins can exist anywhere. They can be placed into the app's bundle without necessarily breaking its signing seal, they can exist within the app's container if the app has a way of copying them in (e.g. it advertises a file type, like Aperture's ".ApertureExport" bundle, so the user can double click to load the plugin, or it provides an "Install Plugin" feature somewhere), and they can exist anywhere in the user's home dir or system volume (if the app requests an exception to read files from those known locations).

No code loading restrictions are placed on signed and sandboxed apps. Any signed and sandboxed process is free to map in any code, though that code is bound by the parent process's sandbox restrictions.

This is the one I am referring to:

2.16 Apps that download or install additional code or resources to add functionality or change their primary purpose will be rejected

So I guess that you can have a plugin system as long as you don't provide an in-app way to download and install them, like Adium does?

As far as I know, yes, that is the case. I'm not an app store reviewer, though.