As for REX, as dnautics said, we've been mostly focused on running raw compute kernels on the current simulated versions (software and FPGA), and for the soon to be in hand silicon (coming this fall)... one of our projects internally is to port the L4 microkernel, and a telecom focused RTOS, but that is as far as our operating systems plans go for the near future. I'd also love to get a Plan 9/inferno demo running on it for fun, but we've got more important work to do at the moment.
The point about the OS related to the Linux-based one for Sunway, but maybe it only runs on the management processor anyhow, with just offload to the others. I'd commented in that respect that we really don't want something like Linux in an ideal world, so I'm pleased to see mention of L4.
Thanks for the comments, and good luck.
One of the things I wondered about is how Linux is adapted to such an architecture. I couldn't find anything from REX on operating system support.