Hi Ed,
what about adding the job to the Quartz scheduler which kicks of the process? Meaning, the responsibility of the job which executes in Quartz would be to either create a knowledge session or get one from somewhere to start a process. This way the schedules are managed by Quartz but what happens when the job is executed is managed by JBPM. Then you can also call JBPM to run the process on an ad-hoc basis, without having to handle timers that should not fire in the process.
Would that work?