I do this, and for the same reason - pagers. I can't guarantee that its the most efficient or "correct" way to do it, but I just have a higher ranked policy in my Outgoing Mail policies which duplicates my normal outbound policy except for the content filter which adds the footer. The modified policy is easy enough for us since our pagers are all in a different email domain, I just have it apply to any recipients in the couple domains our pagers reside in.