Ugh, they still haven’t fixed this? I recommend you file a bug report
https://bugreport.apple.com/. And next time you are in an Apple Store, tell the employees that this is a serious problem impeding your workflow.
Anyway, on my 2007 MBP running Snow Leopard there is a mode very similar to clamshell, but with the laptop open. The built-in display is off, and the picture only goes to the external screen just like clam-shell. The only differences are that the laptop is physically open, and it continues to work even if you unplug the power cord.
I have a distinct feeling I’ve heard that Apple eliminated this mode some time ago, so it might not work on your machine. I mention it because there are two ways to enter this open-clamshell-mode: one is from normal clamshell mode (with power cord in) where you open the machine and then you can unplug it.
The other is, with the cord not plugged in at all, and the laptop displaying to both screens, I am able to, with precise timing, close the lid then quickly open it again, then just as the waking-up sounds begin, close it again, then a moment later open it again, at which point the picture goes entirely to the external screen and the laptop is in open-clamshell mode.
But again, I think Apple killed this behavior. The ideal should be, to have a checkbox in System Preferences, either under Displays or Energy, to the effect of “Allow clamshell mode on battery power”, and another for “Exit clamshell mode when lid is opened”.
They could set the defaults however they like, but those of us who like clamshell mode, and may or may not want to use the built-in keyboard and trackpad for it, would have that option.