It is a pity if Firefox's connection handling can't be reused, but if that's the case it might make more sense to have a separate proxy configuration options in SamePlace. Implementing the FindProxyForURL function would also require implementing about a dozen other predefined functions as per the document in http://wp.netscape.com/eng/mozilla/2.0/relnotes/demo/proxy-live.html. I'm not sure it's worth the effort- Firefox might need to connect to an unlimited amount of hosts, while SamePlace would only need to contact as many servers as there are connection accounts.
If the predefined functions at least can be reused from Firefox, then it might be worth to reconsider. Any opinions?