https://code.google.com/p/chromium/codesearch#chromium/src/p...
and
https://src.chromium.org/chrome/trunk/src/ppapi/c/extensions...
I haven't looked deeply, but both seem to allow for access outside the sandbox. Maybe a chromium committer could give more detail on the safety of the dev interfaces. They are blocked from public usage for a reason though.
PPB_Testing_Dev is a set of helpers for writing tests: querying status of the plugin; running nested message loop to wait for results of async operations; simulate input events which are received only by the plugin itself.
PPB_Ext_Events_Dev is for registering/unregistering events provided by Chrome apps apis (http://developer.chrome.com/apps/api_index.html). It will only be accessible when the NaCl module is included in a Chrome app. This interface hasn't been implemented yet.