It sounds like you might want to:

1. Create a hidden field to store the associated "role" name
2. Add a change event to the "LOC" selection field that populates the hidden role field
   • Use essentially the same logic as in your example, just do it within the form. Maybe use a switch instead of if statements.
   • Make sure to trigger the "change" event on your role field after populating it
3. Use that variable in the filter 
   • $result=team.findMembersByRole({/dataset/role_variable});

Scratch that. I tested it and it looks like you can indeed do that completely within the filter, and you were already pretty close with your example (I used a switch instead, but it should work either way).

switch($util.getValue('location_field')){
    case 'LOC1':
        $result = team.findMembersByRole('Approve1');
    break;
    case 'LOC2':
        $result = team.findMembersByRole('Approve2');
    break;
    case 'LOC3':
        $result = team.findMembersByRole('Approve3');
    break;
}