Can you add a 'select all' option to a webform_select_options_list?
The Checkall module enables this functionality for any checkboxes form element.


  This module provides an extension to the builtin checkboxes element
  that allows developers add "check all" / "toggle" / "uncheck all"
  actions to them. This is specially useful for checkboxes that have a
  lot of elements.


A hook_form_alter() on your webform with this module should get you there.