Hey dude - currently investigating this myself. I don't believe it's possible to easily rig the factory system to display PS2 or other video inputs, and those who've tried have ended up finding it easier and cheaper to just install an aftermarket system into the same space that the Japanese system is in.
Having said that, I have been doing some research and I know of one place who is producing a map DVD/CD that is apparently going to be compatible with the Altezzas. Not sure if it'll pan out, but I'm not giving up hope yet! Sure, the interface will still be in japanese, and not all the features will work, but it's a pretty cool possibility.
There is more info on the NZ Lexus Owners Club forums (
http://nz.lexusownersclub.com/forums/ ), so take a look there, search the forums.