Well, one thing that might be of use is this:
This means that the Debian repositories can be versioned by date, which means that if the build scripts use the build/release date then the builds should (theoretically) be reproducible.
I’m very interested in this (been a Vyatta → VyOS user for a long time, as well as an EdgeOS user) so when I get some time, I’ll take a look myself. In the meanwhile, I’m just throwing it out there, in case someone gets time before I do.