How can I set up DNS for Bonjour and AirPrint to advertise printers across subnets

I have solved this problem by allowing a selected group of the guests to be on the same LAN as the printers; however blocking them significantly with firewalls so that only the printers and a few other machines needed, can be reached from that specific wi-fi network.

If you see that I have provided this answer to my own question; I'd still love to hear how to configure DNS for Bonjour in the current versions of OSX and Server, and otherwise forward Airprint or other protocols as examples.

You need to study and research an internet standard (described in RFC-6763) called "DNS Service Discovery", short: DNS-SD.

All further info you need for that is hosted on DNS-SD.org and on ZeroConf.org.

There, have a look at "How to set up Wide-Area AirPrint". Draw your conclusions about how to achieve the same with AirPlay. (Hint: the DNS service records are differently named...)