Hello @rreignier,
After your question about board to computer communication, I made a proof of concept.
Now if you read the README, you should find a way to make a (RIOT) native ros2 node communicate with a node that is embedded on a microcontroller.
If you want to try this example on your hardware, I would be happy to help you if there is any problem.
Just a little warning : the microcontroller version seems to crash because of a bug I introduced recently. I need to work on that, so if the app complains about heap allocation problem or goes crazy (printing junk on stdout), it’s a known bug. Just don’t control your motors with that for the moment 
!