Monitoring and processing multicast frames?

[epoch]

I am looking at a LAN that uses a 239.X.X.X multicast address to convey alarms generated by users or machines.
I thought I would get advantage of OPNsense's reliability and great GUI to log and display alarms the firewall hears passing on the LAN interface.
Of course looking into the frames and parsing alarms details (sender, cause, scope) would be great.

For now, the only thing I have in 17.7 is a floating rule that matches the address and UDP port, and logs the hit in the firewall's log...

Lets say the final goal is a public access monitoring page, showing alarms details, with some alarms hooked to email notifs (etc.), and log retention for 30 days.
How would you do it in OPNsense?