It’s a possible approach.
Another I would probably use is to use 1 association to the user filled when the user request the assignment and a status field with some options:
User can only request those tasks that have status 1
User presses the button and the user is associated to the task and status is set to status 2
Admin has a list with an xpath on status 2
Admin presses button accept to change the status to 3
Admin presses button to reject and changes status to 1 and sets the association to empty.
This would leave you with a cleaner domain model IMHO.