If you're willing to spend money on this, I can recommend you DVDRegion+CSS Free or DVD Idle Pro (not the non-pro version). I've used them sucessfully. If you have this app running before inserting the disc in the dvd unit, your unit will not change region but still play the disc. It even works if your unit is already locked on another region than the one your disc is meant for.
Here's the page:
DVDIdle.com