I've been doing research too...and here's the most promising
There is a product called CAN-view that is made for Toyota Prius to allow for extra functions and viewing of data from the Hybrid system. The unit can output touch screen coordinates (through a serial port) from the stock Toyota touch screen (which should be the same as the Lexus one) because it interfaces through the Navigation computer connector.
This person has installed a Mac in his Prius and able to control it via the stock touchscreen using the CAN-view unit. http://www.kusnetz.net/prius/
The creator of CAN-view says that if the Prius communication system is the same as the Lexus, then it should work...and we can use the touch screen coordinates coming out of the serial port of the CAN-view...I'm almost willing to bet that it should work since the Navigation computer and connectors looks almost exactly the same.
I don't have the money to try this out at the moment...but maybe it's worth your while to check it out if you really want this to work.