The Resource Usage and Task Planner views are both resource usage views with assignment data. The data you enter into a custom field in the task view is not the same field on the resource side. For example: You add the Text1 field in the Gantt chart view and code tasks. If you then add the Text1 field to the Resource Usage view, you'll see it's blank.

You may be able to get what you want through VBA code to copy the information from a task field > assignment field - but you still won't be able to filter for just the assignment data.