This may be just silly, but I like it so I implemented
it. I taught Middleman how to serve its own proxy.pac,
complete with dynamic PROXY list and testing for the
browser configuration address. This enables me to do
much more sick and twisted things than Mozilla allows
and still only running one "webserver" on my machine.
The gist is that it provides the proxy.pac header
(function decl, proxy port list variable, simple host
checks, etc) and you provide the body via the
[well-thought-out] template named 'proxy.pac'. If
Middleman receives a HTTP request (in contrast to a
proxy request) for /proxy.pac, it will happily deliver one.