This is not possible within the jca architecture. It seems like a good idea though.
You could write your own connection manager (or modify a jboss one) that had that behavior and deploy it in jboss: this would be pretty easy in 3.x but I hesitate to recommend it in 2.4.x.
Your clients will get some kind of exception until the service is running again, right? So how about just setting the idle timeout to less than the time it takes to restart the service?