As I understand it, one use for castellated holes is to allow one PCB to be soldered to another PCB. If the Moteino MO board has a transceiver attached to the bottom side, it would seem that the Moteino MO board could not be directly soldered to a user board. Am I missing something? If not, how might the castellation feature of the Moteino MO boards be used?
It could, you just need a square hole in the target PCB if you have anything mounted on the M0 belly.
I posted a Moteino.lbr Eagle library here (https://github.com/LowPowerLab/Moteino), where you can find a Moteino_M0_LONGPADS layout that has the required radio cutout, you would just drop this in your design.
(https://i.imgur.com/IkLGl4S.png)
Thanks, Felix, I'll take a look.