I'm one of the developers on the GRiSP project.

Our main difference is as you say, that we combine Erlang with RTEMS to make the Erlang VM essentially be the kernel (no Linux running underneath). This let's us use Erlang's inherent soft real-time properties more reliably and allow us to do more hard real-time things.

Our low-level drivers for accessing hardware are written in C, and high level drivers are written in Erlang (for development speed, readability, testability, line count etc.) but can of course be written in C if you need the performance.

I'd be happy to take questions about the project.