Tech demo: Random prop manager editor plugin
The groups and child groups now work. This allows randomised prop setup (as demonstrated in this video, showing base groups and detaill/child groups), and also allows (non random) placement of doors and windows, automatically spawning them in and out, just like the props.
Most likely I will bake in a random selection of props into an array in each blueprint instance in finished maps, so there is no look-up time when spawning the props in. This shouldn't make too much extra data, and it will automate furnishing a large number of buildings. Custom prop placement can be (and will be) done on top of this system of course.