I just ran across Philomel Follows After, by Mabel Esther Allan, which appears to be, well, not quite fanfic of Broster, but sort of related. '[W]ritten when Mabel Esther Allan was fifteen and was her second full length novel. A girls’ school story with a setting inspired by D. K. Broster’s Jacobite trilogy, it shows her early passion for Scotland – or how she imagined Scotland to be – with its clans and tartans, Gaelic language, supernatural folk-lore and dramatic scenery. She later noted, “I seem to have been living in a peculiar world of my own.” '

I'm guessing it's probably terrible (though Allan became an accomplished author), but might be rather interesting to see.