Michael, looking at what you described here is not really a question about task server but more task client. Process engine can utilize existing implementation of task server to maintain task life cycle and your portal solution would be more to provide presentation layer for your users. Whenever user makes an action, task client should be used to inform task server about what happened and then task server notifies process engine to continue with the flow. Task client should be used to get task list, task details (including variables, etc) as embedded component of your portal.
That means that you need to pass variables back and forth between portal and task engine. One interesting addition to that is smart tasks by Mauricio, Demian and Esteban (apologies if I missed someone involved in that project) that provides abstraction layer over jbpm task client stuff (and mouch more than that).
HTH