I am interested in a solution for Squid Proxy to manipulate some html content. Eg. Add some arbitrary HTML/JS code to any page that is requested through the proxy. Is this possible ?
I think you are looking for ICAP or ECAP. I have worked with proxies but have not done anything with either of these yet.
Content Adaptation (ICAP, ECAP, and others which are not as promising in my opinion) http://wiki.squid-cache.org/SquidFaq/ContentAdaptation