http://i.imgur.com/tDG66wt.jpgHere's a quick mockup I made. I just made some guesses on the Trezor's actual dimensions.
If a user wanted to use a wifi/NFC/Bluetooth + battery + USB host device and connected it to the Trezor like the above picture then any arbitrary communication protocol could be used to send information to the red "base station" from the merchant. Then the device could build the transaction without giving the Barista the ability to see the entire financial history of the wallet (depending how smart you could make the device). A cell phone could be linked to such a device as well.
This fixes the problem of the direct electrical connection, but does introduce other security issues (spoofing wifi/NFC/Bluetooth, etc.).
