TIL that the #FreeBSD syslogd expects incoming log messages to come from source port 514:
validate: dgram from IP 10.0.0.2, port 42461.; rejected in rule 1 due to port mismatch.

The #Illumos syslogd isn't so bothered with which source port it sends from.

The fix, make it accept any and all src ports:
syslogd_flags="-a 10.0.0.2/32:* -vv"