web_proxy does not work with .pac files

[christopher-lea-eu]

I'm trying to make my client application proxy aware, and using WinHttpGetIEProxyConfigForCurrentUser to get the proxy configuration before creating my HttpClient.
In many of my users cases this means lpszAutoConfigUrl = "http://something.com/proxy.pac".

However, it seems Casablanca does not handle this how I would expect. The .pac file is not parsed by casablanca and no correct proxy is subsequently set, as far as I can tell. A quick look through http_client_winhttp seems to suggest I can only use a direct proxy or WPAD auto discovery via DNS/DHCP?

How can I configure Casablanca to correctly use a .pac proxy?

Thanks,
Chris