I haven't seen any Plextor-specific Region-Free firmware (then again I wouldn't know where to look), but you could at least try AnyDVD HD. I'm assuming that Regions are one of the many problems it overcomes, since it's constantly updated for Blu-Ray copy-protection variants. I bought a lifetime license for cheap (compared to today's prices) a day before Obama's inauguration (the end of the old pricings), but I do not yet have a Blu-Ray drive or HD-DVD drive to exploit with the software. What I do know from experience is that the nonHD version of AnyDVD has overcome copy-protection for me with newer standard-definition DVDs when no other software could, otherwise I would still be solely a DVD Decrypter guy.
Check out the AnyDVD subforum of Club CD Freaks, and it can probably answer any questions you might have about the software.