So it turns out that i didn't read the datasheet carefully enough. A "quick" look showed that the SAMC21Gxxx was exactly the same as the SAMD21Gxxx, except for the 5V-ness. All the pins were in the same place, and etc.

But it turns out that none of the SAMC parts have a USB controller! SAMC21 has CAN on the same pins that SAMD21 has USB. I guess I can't blame anyone but myself, but I wish Atmel had used more distinguishing part numbers. Sigh.
(Found while looking at SW. I guess the boards are still usable.)