The touchscreen coordinates are sent over the entertainment system bus. This is variously called the AVC-LAN by Toyota, the IP-BUS by Pioneer, and the IE-BUS by NEC. Even the wikipedia has documentation on IE-BUS, so the low-level protocols are well-known, and you'll have to sniff the high-level ones. Use of the Prius IE-BUS has been achieved by devices like Ultimate Lockpick (Coastal Technologies) and CAN-view, but they're not telling us what they found. The list of AVC-bus IDs is in the service manual,
You can't passively monitor for touch-screen coordinates, you have to register to get them, or they won't be sent over the bus. The navigation system is known to use them.
For more information, see the AVC-Lan paper