I understand it may be possible via sl4a python script.
I am also trying to figure out how to communicate between my droid and my arduino (but over bluetooth). What I've done is built a simple android app that communicates with the arduino via bluetooth (or in your case, usb) and use tasker to emulate button touching in that app. It's a dirty way of doing it, but it works. Hopefully someone is working on integrating Arduino and Tasker somehow.
Love the idea over USB serial as it will power the arduino device.