Agent-Idle state as a build trigger

As a bamboo user with intermittent test failures, I'd like to run additional test runs when there is nothing in the queue for an agent, or there hasn't been anything in the queue for a long time, so that I can take advantage of agents on idle.

Builds that are executed from this trigger would by default have lower priority than any other build in the queue available for the agent.

I need to be able to define parameters for the idle time, like how long it is before a build is idle for this use-case. I need to be able to define this as a special trigger for a given plan.