URLFetch behind a Proxy Server on App Engine Production
You can always roll your own:

In case of fixed destination: just setup a fixed port forwarding on a proxy server. Then send requests from GAE to proxy. If you have multiple destinations, then set forwarding on separate ports, one for each destination. 
In case of dynamic destination (too much to handle via fixed port forwarding), your GAE app adds a custom http header (X-Something) containing final destination and then connects to custom proxy. Custom proxy inspects this field and forwards the request to the destination.
Is there a way to specify a proxy server when using urlfetch on Google App Engine? 
Specifically, every time I make a call using urlfetch, I want GAE to go through a proxy server. I want to do this on production, not just dev.
I want to use a proxy because there are problems with using google's outbound IP addresses (rate limiting, no static outbound IP, sometimes blacklisted, etc.). Setting a proxy is normally easy if you can edit the http message itself, but GAE's API does not appear to let you do this.