The Bell Labs MERT system did almost the same thing: its low-priority process was a Unix emulator along the lines of MS WSL 1.  See <https://en.wikipedia.org/wiki/Multi-Environment_Real-Time> for basics and links.

Similarly, the PDP-8's modular real-time system RTS/8 had a symbiotic relationship with OS/8, the single-user operating system; you programmed and built instances of RTS/8 under OS/8 and then booted them, but if you had enough memory, you could include the OS8 [sic] task in the RTS build and it would run OS/8 as the lowest priority task.

John Cowan          http://vrici.lojban.org/~cowan        cowan@ccil.org
This great college [Trinity], of this ancient university [Cambridge],
has seen some strange sights. It has seen Wordsworth drunk and Porson
sober. And here am I, a better poet than Porson, and a better scholar
than Wordsworth, somewhere betwixt and between.  --A.E. Housman