Get a consultant/partner (Score 1)

There are groups out there that make a living off of introducing large organizations to open source. Get in touch with one of them. These guys understand the issue from management's perspective and know how to pitch to execs. You'll get the open source you want and management will get the checks & balances, best practices, policies, references, etc. that they need to feel warm & fuzzy. If you are going to go it alone, I strongly recommend starting with a policy & audit strategy as mentioned previously in this thread.