(A couple of people have recently "liked" this project, so I thought I should post an update...)

I'm ready to call the existing PCB design a failure.

Aside from the non-presence of USB in the chip, the PCB was designed with the "sweet spot price" prototype PCB of 50x50mm.  Since then, most of the cheap vendors have increased the max size of low-cost boards to 100x100mm, which would easily fit a much more "comfortable" design (Arduino Shield compatible, for instance.)

I haven't gotten around to actually trying to build one of the boards (scary QFN pinouts!), and now I'm not even sure it's worth trying :-(