Has anyone come across any product or technology that can handle the multi-path-ness and the private-network-ness like a regular router, but also provides the intelligent per-flow path steering based on e.g. latency, like an SD-WAN device (and/or some firewalls)?

Maybe add a little bit of linear optimization on top of faucet/openvswitch/openflow to calculate best paths based upon bandwidth, paths, and fill-factors.  There is a presentation where Google uses that technique to obtain high utilization on their links (not necessarily those tools though).

Raymond Burkholder

This is what a large Italian wisp has done, here are a couple of presentations made at our ITNOG sessions.

I’m not sure if they have open sourced anything though.

https://www.itnog.it/itnog4/files/14-Traffic%20Engineering%20-%20the%20EOLO%20way%20of%20life.pdf

https://www.itnog.it/itnog3/files/ITNOG3-EOLO.pdf

Brian