I do something like what ctwaley suggests.

My current employer uses a PAC file to select from various web caches, or go direct. This is how I do it:

I download the companies PAC (cache.pac). I edit it and change the line:

function FindProxyForURL(url, host)

to:

function otherFindProxyForURL(url, host)


Now, I edit no-ads.pac. At the end of the, I include the contents of cache.pac (with the above change). Then, for each instance of

return normal;

I change it to:

return otherFindProxyForURL(url, host);


There is actually a way for Gecko-based browsers (Mozilla, Firefox, etc) to do this internally, but it doesn't work for IE or Opera. I will look into it again sometime.