It is possible, however you have to roll it yourself. If you only have a few instances that are using the ThreadPool you can wire up org.jboss.seam.async.ThreadPoolDispatcher in components.xml under a different component name, inject in into a component and call it manually. Not ideal, but it will get the job done.
Thanks for you reply - I was wondering if anyone had done this before and seen significant performance gains. I will have to run some JMeter load tests to see if there are any significant advantages otherwise.