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The software on the DVD asks the player what region it is. If a player responds 'region free' (i.e. no region is set) the disc will not play. However, if the player is set to the correct specific region, the disc will play.
It all depends on exactly how Region X works, but as all the talk on the web site etc. implies that it actually sets the required region, then it should be okay.
But don't quote me on any of that!
Most DVDs don't use RCE anyway as the vast majority of DVD distributors aren't bothered by importing anyway.