Hi Milo,
You can disable a pool member from an iRule using LB::down, but you cannot set it offline. So the monitors would continue to poll the server and mark it back up on the first successful response.
If you want to give some users the ability to disable a pool member and they have access to the servers themselves, you could configure a monitor which checks for a specific string in the response content for a page. The users could then change or rename that page when they want to take a server down. Once the maintenance is finished, they could restore the page being monitored.
If the users don't have access to the servers, you could create an iControl based program which allows users to disable specific pool members. See the iControl wiki pages for disabling pool members for details on this.
Another option would be to give the users limited access to the LTM GUI. Using partitions, you can limit the objects they can modify to specific VIPs and pools.
Aaron