Solved it, I’m posting here for completeness sake:
Using mdns-scan on Linux I was able to probe my LAN for mDNS addresses.
I noticed mt-daapd was missing from that list.
Attaching a system call traces (truss) to mt-daapd and the mdnsd daemon on Solaris showed that they didn’t even receive the query packets.
Running a sniffer /magically resolved the problem/.
After a post on the Solaris forum, the issue was identified as a driver multicast filter bug, and a fix was suggested by the driver developer.
Driver was myk-2.6.1.