[Jekyll]

I don't know if I'm missing something here... but what's wrong with running your laptop directly off the mains without a battery and unplugging the cord if necessary so the computer shuts down?

[Pwntastic]

There are tools available to law enforcement that allow them to splice into a power cable and convert it to battery power while still online

edit: i found it:

http://www.cru-inc.com/products/wiebetech/hotplug_field_kit/

[kyleknighted]

The concept is sound, but new MacBooks do not have removable batteries.