First, you could create a separate domain for the high-risk areas, which would have other advantages as well, as they could manage other differences (length of password, history, account lockout, etc.). Second, while there is only one enforceable password expiration policy per domain, there is no reason they cannot procedurally insist on their group passwords being changed at whatever schedule. They also could write scripts to check on this and possible e-mail users who failed to follow the policy, or make some other change that would be effective in enforcing the policy. Of course, they might also write custom software to enforce more frequent password changing.
This was first published in April 2003