I am almost certain there is no way to attach any listeners to the built in re-assign event. It simply updates the action pane.

To do this right now, you would have to build your own re-assign element. It could consist of a drop down where they can select a user, and a re-assign action button added to their task. That would allow you to perform actions between the re-assign, including updating the assignee with workflow and returning to the user task. Even this requires a direct call to the Forms database though in order to retrieve the users list for the drop down.

I don't like using lookups to self-reference the forms database, since the schema could change. The expectation that forms knows who it's own users are is so common though, this is one I am starting to be OK with myself.