Currently, the default behavior of sort_column or sorting of pages within the wp_list_pages() function... (assuming you don't modify it) is by page order. If all equal, then by page title.
But if you use sort_column=menu_order, it's by menu order EXCEPT it DOESN'T default to page title (a-z) if all pages have the same order (eg. 0)
Even if you give your pages 1-26 values corresponding to A-Z, in the internal groups (eg. A(1), B(2), etc), they don't sort by page title. AND there is no way to use a dot.decimal number system or something other than numbers for ordering (eg. letters).
Given that the default ISpage orderr, then page title, defining sort_column=menu_order should NOT break the other default behavior of using page title when the order values are all equal OR when, giving 1-26 values, the ones equaling "3" don't sort within by page title.
It's just broken. And in most cases, a site owner wants to define the order of sub-menu items (inner pages) explicitly... but when you need an index (as in a list of partners, A-Z) then you WANT the page title functionality.
If there is a way to do this, please post it here; if not, I'd love to see some way to fix hat seems pretty broken.