Thanks to Four Short Links, I found out about the Zephyr Project.

I'm passing on this information because it could be of interest to some Teensy 3 (and above?) users. I have no connection to Zephyr. I would not have posted this information if Zephyr were not backed by the Linux Foundation.

Since Zephyr doesn't support Teensy 3.x (yet), this is just F.Y.I. and something to possibly keep an eye on.