I have a satisfactory workaround for machines that are predominately idle: use BitcoinMinerSaver ( http://bitcointalk.org/?topic=8385.0
1) Set up BitcoinMinerSaver on all user accounts.
2) Install TweakUI 2.1. Under "Logon", there is an option to change the default screen saver for the logon screen. Use this option - it will then copy all screen saver options (which will be the Bitcoin miner) of the current user as the logon screen saver.
Note that step "2)" is required because if users simply "switch user" instead of going idle (and letting that user's screen saver take over), then the default screen saver takes over rather than the user's screen saver. Step "2)" changes the default screen saver so that no matter which screen saver activates (User1's, User2's, or the Logon screen default screen saver), the miner runs.
A) The BitcoinMinerSaver only works with one device.
B) Only works while idle.