When I have to do this, I make sure all notes at the root are fully collapsed. I add a note to the top of the root outline (say “Top Node”). I select all the notes except Top Node, the press tab to make them Top Node’s children. Set the sort for Top Node. Select all of Top Node’s now-sorted children. Press shift-tab to un-indent them, back to the root. Done.
Sort of a 5-star kludge, admittedly.
I think a problem with “sort at the root” is “sort what?” I have lots of notes that I use as containers for tools, and I always want them at the bottom of the root.