If anybody still cares, Karel provided me with the script he mentioned. From there, I wrote up a blog article and a deeper dissection of getting AVB devices (specifically MOTU) working with my S3L, as well as some of the tooling on macOS that helps with AVB channel routing.
https://blog.forestent.com/2019/05/1...y-avb-devices/
I don't [currently] have access to an S6L to try similar out, but perhaps the work I've done could help somebody else.