What you're wanting to do is the exact same as clamshell only with the lid open. To do that, you'd have to put it into clamshell mode first and then open the lid for the MacBook's internal display to be off and the external display to be on. Hover, as I said, it's essentially still the same as clamshell, and you might as well be plugged in unless you want to drain your battery.
Wow. Why do Apple's notebooks' display options blow? Even on my ancient Dell Inspiron 8200, I could just plug in a projector, monitor, whatever, hit Fn-F8, and it would display. Then I could set the signal output to be a certain resolution, and when I unplugged and hit Fn-F8 again, my old laptop resolution would still be intact, and the screen would come back on.
It also didn't affect any sleep issues or anything like that. It was just basically like the laptop was a desktop PC running a monitor.
Oh well. I suppose when I get the monitor I'll fiddle with it and see what works and what doesn't.