To install, just drag it onto your browser toolbar or right-click it and bookmark it. I have only tested it briefly in Safari 2.0.4 and Firefox 2.0.0.7, but it seems to work. Note that if you haven't authenticated this session, the proxy will first send you to a login page. It could extended to support other UH proxies, but bookmarklets can only be so large.
The source can be found here