Where is this part of the project? I'm not really familiar with USB from the stack perspective, so I'm not sure how to contribute to this issue. :(

this will come in time: we need to check the existing Arduino USB device stack and make it more efficient, if possible, or rewrite the foundations: the famous chapter 9 and right after device classes drivers

Not having it is somewhat painful for me. Would you object to me standing it up as it is and refactoring it for speed later?

Yes, I object doing something quick and dirty :-|

But, to let you move without pain, I just pushed as-is the USB device stacking coming from Flutter Wireless port (https://github.com/flutterwireless/ArduinoCodebase). This port isn't really what I would like to see here but at least it is working on SAM3S1 and should be 'easy' to use on SAM4S.

Play with it, fix some wrong #include, don't forget to add all .c and .cpp files in your variant makefile and you should be ok for a while.

Branch is here: https://github.com/aethaniel/ExperimentalCore-sam/tree/usb_device_from_flutter_wireless