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.
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