This weakaura will automatically promote all members of your battleground group to assistants (if you're lead). Perfect for people who don't happen to notice that they're battleground raid lead. Only operates during the 2 minutes before the battleground actually starts.
Toggling the built-in "All Assist" option is unfortunately not possible programmatically, so this promotes people individually, but this also gives you the option of demoting an individual person without having to demote everyone.