Currently, the programmer working on the project wrote a Firefox plugin that allows "leaving things on pages". The items left on pages are stored in a database and queries are done via RESTful calls.
This is how it's done.
The idea of looking for someway to intercept the requests is to make the game "browser agnostic" so I'm researching what ways are possible.
A proxy works, as long as everyone connects through it. HTTP defines SOCKS protocol for proxies. They must be manually configured on each client that wishes to use them (making them useless for most intranets and corporate networks which use such proxies internally). A transparent proxy would also need to respect HTTP request properties to avoid caching issues.
In lab, for experiment, sure. For WAN users - not viable.
Third option is writing plugin in something like Greasemonkey, available in all browsers.
Final option is having a gateway. Users go to www.redirector.com and specify they want to visit www.foo.com. Instead of making direct connection, server requests it on their behalf. Upon receiving the result, it rewrites all links on that page to again return results via www.redirector.com.
Downside of such approach is it doesn't allow for SSL and may cause problems with cookies.
Page operators will also be alerted to such traffic and may choose to blacklist such a server, since it's one of the simplest MITM attacks. Such portals are also closely associated with spam relays and other undesirable traffic and the originating IPs get blacklisted. Common term for such technology is anonymizer, most popular sites block them and monitor for such traffic.
I was not aware that some technologies that can do the necessary functions could be considered dangerous.
If such technique were possible transparently, online commerce isn't possible anymore. That is how dangerous it is.
Considerable effort goes into making precisely such behavior impossible, so there's some technical obstacles as well as many business processes in place that control such behavior.
In lab it's trivial - just connect to a proxy. But having something like this accessible on open web is, hopefully, very difficult.
Finally, outside of technical issues, such rewrites violate definition of net neutrality. Net neutrality basically states that anyone relaying traffic not intended for them will not interfere with it in any way, either through traffic shaping, rewriting, by modifying it or otherwise. It's not exactly a law, but is foundation of open internet. Topics like this are quite important given recent developments around SOPA and such.
SOPA, for example, gave individuals precisely this ability - they allowed them to intercept anyone's traffic and return their own results.