With R5 and VMPro you can accomplish this task with relative ease. When complete you will have one short code that is dialed. This code will connect to a VMPro Module Start Point that has a Menu. Press Menu Option 1 to turn forward on or Menu Option 2 to turn forward off. Menu Option 1 and 2 is then connected to unique Generic Actions. One Generic Action for Forward on per user and one Generic Action for forward off per user. The actions are chained from the previous Generic Actions Next connector. So if the user selects to turn forward on then it is applied to each user one at a time. As long as a forward number is set per user then Forward Unconditional will be applied.
Dress it up by programming Forward All Buttons for each user. Record Menu wav that says something like " Press 1 to turn Multi Forward On or press 2 to turn Multi Forward off"