I wanna suggest a plugin. With that plugin we can show ads at top or bottom of all posts and pages. And the ads will only show users arrived via a search engine.
There are two plugin that have this features but they must recode and must work with wpmu. Can you combine them?
Plugin1: Smart Ads: With this plugin we can show the ads at top or bottom of posts and pages
Plugin2: Ad Widget: Show the ads users arrived via a search engine