USB-device on SAM3-H256 - advice?
You'd better check examples bundled with this board. If it have any examples on HID devices (i.e. keyboard, mouse) - you are good to go. If there are no working examples - it might be hard to figure it out how to make it work from scratch.

Personally, I've implemented USB-keyboard emulator on V-USB library: http://www.obdev.at/products/vusb/easylogger.html - it works on nearly any atmega device. So you can for example get Arduino board and go from there.