Redox (https://www.redox-os.org) is still under heavy development, and isn't intended for widespread use yet, but it is based on a microkernel architecture. I don't know if that's close enough to what you'd like to see.

Isn't that made for somewhat beefier systems? Wouldn't Linux fill much of that niche today (modulo RT properties)?

Remember that a lot of embedded systems run on really constrained chipsets where memory is measured in kilobytes and where the underlying architecture isn't necessarily a von Neuman architecture.