If you mean lappy as in laptop forget it. Even if you could manage to get a 12V tap off the power supply, they use power supplies with minimal ratings to conserve battery life. We're talking about running a battery-less passthru off a desktop computer power supply, not a laptop power supply or USB port. Running a battery-less passthru off of any computer USB port is a really bad idea. Some can actually handle it but with a lot of voltage sag. Most will either shut down the port with or without warning or the port will burn out possibly taking the whole motherboard with it. For passthrus with a battery in them, a computer USB port is usually okay, but a wall wart or other dedicated supply is still better.
If that's not what you were talking about, then nevermind.