You can do something like
where actorId is the id of 'User X'. Loop over all taskInstances and set the actor to null so it is returned to the pool or set the actor to an explicit one.
btw, the seam source is a great source of inspiration.
Ronald, thanks for your quick reply! Very much appreciated.
Right after I posted the question, it did occur to me to dig into the Seam source to see how Seam was accessing the task list. We have tried the solution you suggested and it's working great.