PeterVH, thanks for the pinout details. I have updated the image in the first post of the thread.
I looked around for SWD breakouts and debuggers, did not find much. I found more i terms of breakouts for the full JTAG header, such as bus blaster which can be used with urJTAG. There is also a SWD library.
Most of the software I came across was pricey; this hitex serial wire viewer gives an idea of what can be done with the SWD interface. goJTAG looks interesting, with free software for Windows and Linux, and free training materials. It seems to use the picoTAP as a hardware connector, but I didn't see this on sale anywhere. However they do provide a schematic so I suppose clones are possible.
It would be nice to see in-circuit debugging integrated into a future version of the Arduino IDE.