How to secure bookcase to drywall right over concrete without drilling concrete?
I think you already hit on it. The bulk head at the top would be a good place to secure the book shelf from tipping. Set the book shelf in place and then angle bracket the structure to the ceiling so it won't tip. Find a stud or other structure at the ceiling to make it sound.

If the book shelf does not stand as tall as the ceiling, then you have another aspect than needs attention. Think outside the box. Can you frame in an addition to the existing, what, a duct? Just think that you won't need extreme support for the bookshelf if you have good connections to the ceiling. For example, if you stand the bookshelf on it's own, how hard is it to tip over? Now think if you screw that to the ceiling, how hard would it be to tip over? If you have kids, or even if you don't, make sure it's secure.