Wingco,
My guess is that applying 12v to the power socket that is expecting 19v will not work. But it may, 'cos it was just my guess.
You could probably try it with the lappy battery removed. It's unlikely that too LOW a voltage will harm anything, the worst that can happen is it just won't work. Make sure the polarity is right, though.
If you're very careful, maybe you could get a circuit diagram or pinouts of your battery and identify the +ve and -ve terminals, then apply 12v to the appropriate connectors in the lappy's battery socket (with the battery removed.) I'd get advice on this first, trial and error is NOT the way to go.
One thing about the adapter, though. Even if it uses more juice it may provide some spike protection to your computer that a direct connection would not. Maybe not important if you're using an isolated battery, but if you're plugging into a car electrical system I think you should consider that. Car power systems are pretty spikey.
AA