Both the BugzillaLPPlugin ExternalBugTracker and the TracLPPlugin ExternalBugTracker use XML-RPC to communicate with remote hosts. Unfortunately, the XML-RPC Transport class provided by Python's xmlrpclib (see http://bugs.python.org/issue648658) doesn't do proxying very well, and so neither of these ExternalBugTrackers will currently be able to work in Launchpad's staging or production environments.
To allow them to work, we need to create a custom XML-RPC Transport that can handle proxying. There is a patch attached (http://bugs.python.org/file8213/HTTPTransport.py) to the python.org bug listed above which would be a good basis from which to start, though it would need to be adapted to our purposes (specifically, the handling of cookies in the request and response headers, some of which already happens in the existing custom transports that we've written for these ExternalBugTrackers).
Since the rollout today, the checkwatches script has started failing for bug tracker types with plugin support. This probably deserves a release-critical fix.