Hey robo, I was able to resolve my issues by commiting to solder. I have no idea why, but when i commited the project to solder the hanging magically vanished! Randal had built just the USB portion of the circuit and I believe he didn't have any issues.
Anyways, if you can I would sugest soldering it together if you haven't already as a first step if at all possible. I had narrowed it down to somewhere in the USBKeyboard.h file where it was hanging (don't have my notes infront of me now) but couldn't identify the exact line. I came to the conclusion that there must have been noise or something as while it happened frequently, it wasn't every single time and it was sporatic. It wasn't code related as I pulled all of the code that referenced the switch pin out of the sketch and the MC would still sporatically hang when the button was pushed. As you mentioned, I too tried on multiple Arduino cards (Dumelinove, NANO and even a standalone). Possibly a drop in voltage as the button is closed but without an osscilloscope it is hard to tell what's going on. One thing is for sure, USB is extremely sensitive to voltages.
Solder Solder Solder... That's my three cents! 