Managed to get a simple 2-thread Blinky program running under RTX. The lack of documentation about how to structure the project was the problem. There’s another configuration file, that has to be set up for each project.
Today I discovered that I have to make a new project for each different arrangement of drivers. It ought to be handled with a few compile-time defines, but that would make things even more ugly.
Now I’m seeing if I can create a thread to handle UART activity. that means creating a whole new project.