This is one of the old OSes that I wish had been open sourced. I would really like to see the Pascal source code, and it would be neat to see something that wasn't a UNIX.
One could always look at the Domain Engineering manuals and the data structures described therein and the rest of the APIs and docs and reimplement it. That’s basically what Comer did with Xinu, Tanenbaum did with Minix, Linus did with Linux, etc., just with Unix—and it’s mostly how Aegis (the original name for Domain/OS) came about too.