It turns out that this is not easily done! Hence a new feature request.
In our instance, which utilises group inboxes quite a bit, it is quite common to want to include messages in search. Prepending in:all to a search achieves this, but isn’t something that can be easily taught to users.
Thanks to the wonderful recent work by @zogstrip, this is now possible via the UI. While this is brilliant for our power users, our noob users will take a while to find it, forget to use it, etc etc.
I’d love to have a way to make it the default search for my users (and me!).