Jack is pretty neat, and open source.
It’s designed to be an open source cross platform low latency application to application audio routing program.
Some time ago i was going to toss out the idea of the devs looking at the Jack interface (which I believe is available in Juce) as a mechanism for inter-instance audio routing. But i thought i should do some more playing with it before making such a suggestion, and i never got around to it.
ASIO is not the main goal because it’s a Linux/Mac/windows thing, and ASIO is purely a windows thing. But it does offer ASIO on windows.