Yes. It's fine to do that on the main page. But don't do it on the top bar on the proxified page. Also please keep in mind that it won't get you much.
i don't think there is a problem with putting Google ads on a proxy site...i know a friend who is currently using one ..but thats only on the main page and i believe the earnings are pretty good as well ...
Like said above you can put it on your site but not on the proxified pages, use adbrite for proxified pages. Dont expect the big bucks. Make 50 of those sites and then you can expect a few extra bucks in your pocket. Good Luck