I've been using from quite some time a "search provider" in Opera that lets me do a site-wide search. Since it uses JavaScript, I just tested one thing, and it looks like it could satisfy your request.
Within Opera, go to "Options" -> "Search" and click "Add". Choose any descriptive name you want, and (optionally) a keyword, and paste this in "Address" (third box, if my translation doesn't match the official one):
javascript:void(location.href='http://www.google.com/search?q=%s+site:'+window.location.href+'&sourceid=opera')
To use it, once you are in the page you want to search a word in, go to the URL bar and type: "<keyword you chose> <word you wish to search for>" (without quotes and <> signs, of course), and press Enter. Test it and tell me how it goes