After which the original source can ban your server's IP.
Let the whack-a-mole games begin.
Proxy the script and alter it... Whack. :P
That's pretty much the case for any remote content. Proxy and rewrite the response. But that added step is usually enough, especially for something as complicated as a Browser MMO.
Also, web servers should have rate limits set which would make proxying less viable, even for IP ranges.