I hope that consideration is given to that so that it does not become impossible in the future. It would be much better if we have a single C client library that works on all platforms, with features turned on and off as needed for resources and use cases. Otherwise there is a big risk of divergence.
Thats the goal, really, and anything else is totally not decided on, yet. “urcl” was just a placeholder name. While we initially thought that the architecture layering sketch was good to give a rough idea, I now see it may be interpreted to mean more than it does. Therefore, I updated the repository description at GitHub - micro-ROS/micro-ROS.github.io: A platform for seamless integration of resource constrained devices in the ROS ecosystem. to be a bit more generic 
Remember, this is all open source, and takes inspiration from a lot of work, both the original ROS2 embedded attempts, rosserial, as well as, most recently, the RIOT work by @astralien3000 and colleagues.