Very well spotted, thanks for that…! Indeed my Trellis-install was way behind (roughly December 2016) and #734 was the last commit that was included 
Did that already, all are available via apt.
Conclusion
It’s working now, thanks so much @ned & @fullyint! I had to update my local Trellis and then the apt_packages_custom-directives via main.yml came through smoothly (running vagrant reload --provision to spin the server anew).
Will be rejoicing once I get my local machine running again… 