Relay broadcast packets between subnets

I like to keep things like a smart home IoT network separated from my standard internal LAN. However, things like casting using a chromecast from a phone/laptop on the LAN to a chromecast on the IoT network doesn't work out of the box as it uses broadcast packets to perform autodiscovery of devices.

I currently use https://www.joachim-breitner.de/udp-broadcast-relay/ on a PC connected to both networks which creates a bridge, but it would be really nice for my firebrick to have such functionality built-in so that I can specify a range of ports and interfaces for which broadcast packets would be forwarded between.