SVN and proxy servers
It turns out that although SVN works through port 80, the proxy server I’m behind filters HTTP headers, one of which is PROPFIND. Google tells me that I can configure SVN to use a proxy server, which is what I need to do with several applications (including Live Writer). Although I have to hard-code my password into the configuration file, it fixes the problem:
I have to change the configuration file every few months when my password expires, but otherwise I’m good.