What version of UCCE/UCCX? Do agents use Cisco Agent Desktop?
Supervisors can change the agent states using Cisco Supervisor Desktop. I was also thinking you could use a workflow, but if I think the agent state change action can only be used in call event workflows (like when they hang up a call). Not sure why? Anywa I suppose you could use the Macro action and record a CAD macro (Ctrl W to go Ready).