Well, the second test I built happened to get around it. In Internet Explorer the null byte character injected anywhere in HTML is ignored. Proxyline apparently didn’t take that into account as it’s really not normal to throw null chars in the middle of HTML. Try entering this URL into Proxyline: http://ha.ckers.org/weird/proxyline.html
Adding a few lines of obfuscation to call my environmental variable page “before” and “after” in an iframe help to show that I can accurately de-anonymize any user of the proxy. Null bytes are tricky but I’m sure there are probably other ways around this as well. I don’t think there is any substitute for real proxies, but it’s an interesting service if they can get it working more securely.