I don't know exactly, but I've spent some time with Jbpm/Seam recently and had to solve some issues myself. have a look at org.jboss.seam.bpm.SeamUserCodeInterceptor and method executeAction. It calls initProcessAndTask method that sets process id on BusinessProcess component. This component is used by Seam a lot when intercepting different Jbpm-related annotations.
I guess this method should be called when you invoke your action handler and it should set the component up.
Also have a look at BusinessProcessInterceptor in the same package to see if your annotations are invoked, put some breakpoints in there and have a go :-)