Much faster solution if anyone looks at this.
There is an active FreeBSD port of "mdns-repeater". Its args are a list of interfaces optionally preceded with -f for foreground/debug mode (run -f from console not GUI!), and it listens for mdns on any interface, and repeats them out on all others.
You need to add firewall rules to allow the interfaces to receive the packets (and possibly routing rules depending on config), and voila - airplay minimal version. No need for CUPS if your printer or any LAN s/w can work with airplay, and many modern printers (eg HP's) can do so. Just need to set a system command to start it (without -f) on reboot.
Even if not packaged, the only file really needed is the 14k file at /usr/local/bin/mdns-repeater, so the port can be untxz'ed and that single file used, until a package exists. It doesn't seem to present a high risk of abuse from being outside pfSense GUI control since it has very few parameters (the interfaces which share mdns packets), and is overridden by (and logged by) the firewall.
It could well be trivial to make this into an Airplay-enabler and have it as a standard package for pfsense, or even built-in, as many people have issues with Apple Airplay + LAN printing and discovery, and they're popular enough and the fix simple enough to be worth bringing onboard.
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