You could look at a product that does it on the Exchange Server. Then, whether the client is Outlook or OWA, the disclaimer software would do its thing.
We looked at a product called eXclaimer that was pretty cool and cheap to purchase - something like $99?
MS has a DLL for Exchange 2000 that may do the trick. It's free, but maybe a little trickier to setup. Sorry, don't remember what it's called but I'm sure it'll be up on microsoft.com somewhere...
HTH,
RobbyB