It seems there exists no tool to simply convert one type of tunnel into another. SSH supports both tcp port forwarding and a built-in SOCKS proxy, both of which are incredibly useful. But it lacks a native HTTP proxy. The sad truth is, most applications today only seem to support HTTP proxies or are beginning to support SOCKS. Until such time that SOCKS proxies are universally accepted in networking apps, I need an HTTP proxy for my SSH client.
There are some LD_PRELOAD apps which will overload network operations with their own and send them through a proxy (like ProxyChains). This seems an unportable and more hacky solution than I would use (are you going to change all your desktop links for 'audacious' to be prefixed with 'proxychains'?). I am willing to write a HTTP-to-SOCKS proxy but don't have the time just yet. CPAN seems to have a pure-perl HTTP proxy server, and I can probably leverage IO::Socket::Socks to connect to the SOCKS5 server in ssh.
In any case, my immediate needs are fulfilled: I can use 'curl' to tunnel through ssh's SOCKS for most of my needs. This blog entry on tunneling svn was a useful quick hack to commit my changes through an ssh forwarded port, but much more of a hack than i'm willing to commit to; i'd rather just enable or disable an http proxy.