Use of protocols that are designed for the same local network, be it broadcast or multicast.. Are meant for devices on the same L2 network.. If you have a TV that wants to find your sonos speakers for an example via such a protocol.. Simple solution put them on the same network! Done..
Jimp point here is that what protocols your devices use on some local network has zero to do with pfsense.. If they want to talk UPnP or or SSDP or DLNA between each other you have zero to do on pfsense for that to happen.. If you have some sort of broadcast or multicast protocol you can try out avhai which helps with mdns, etc. Or you can play with igmp proxy for your multicast stuff. Which is most likely done better on your switch setup..
To be honest devices that require such nonsense as having to be on the same L2 to work, I wouldn't use those - vote with your dollars.. Nice that they want to make these things easy for the idiot user to just plug and discover via some broadcast/multicast protocol. Great.. But allowing me to put in an IP or a FQDN of the device it wants to talk to should also be a option..