I can't remember such a feature. Leading spaces and tabs are generally indentation. And pressing shift+tab removes it. If you re-add it, it only adds the indention type you've configured.
As I mentioned, unchecking "Tab key idents", I get desired results, so, in my opinion, it should be possible for multi-lines, also.
And Now I think I remember that shift+tab was the one that used to work for multiple-lines. Anyway, one way or other (tab or shift+tab), it used to work. I'm pretty sure. Or my memory is broken.
Anyway, if I can't do this by configuration, how can I achieve my desired results? Snippet, plugin, local patch-branch?
Besides, I find it weird to even want such a mixed indentation. Why not use tabs or spaces consistently?
It's not pure identation. It's ident + align. And to answer your question - to make code readable. Consider this code (php example):
->->$sSQL = 'SELECT `column1` ' . ->->........' FROM `table` ' . ->->........' WHERE `column2` = (..)
I use tab for ident, space for align.
In this way every developer who works on code can have custom settings for his "tab" key - be it 2, 4, 8 or whatver-count chars wide. And code will be readable for all of them.