I did this in 1998 for executive offices at a company in Phoenix.  All you need is a magnetic door holder which is basically a mini mag lock and self-closing door hardware.

I set it up so the mag locks were always energized.  This way if someone opened the door all the way they would stick and be held open.  Using the door(s) normally to walk in-and-out wouldn't open them far enough to be caught by the magnets.

A push button under the desks would disrupt power to the magnet(s) and the auto closers on the doors would swing them shut.

In this company all the executive desks were built-in (not cubicle stuff) so wiring to the desk was simple and it worked really well.  The CEO had double doors to his office suite and the other execs had single doors.  I think I wired up doors to 5 offices and one conference room this way.

If they needed privacy for a phone call, all they had to do was hit the button and the door(s) closed.  You also knew you were in trouble if you got called into one of the offices and the door closed right behind you as you walked in...

I don't remember the exact hardware I used, but here's a link to the same type of stuff:

[Link: sdcsecurity.com]