Thanks for your response.
I'm aware that iTunes filters traffic outside of its subnet, I'm

thinking a pf.conf(5) rule something like
	nat on enc0 inet proto udp from $subnet_A to 224/4 port = 5353 ->

$subnet_B_gateway static-port
might successfully fool iTunes into not filtering the traffic. This

wouldn't successfully route the packet, however, as my routing table

shows
	224/4	127.0.0.1
which I guess means that multicasted traffic needs the assistance of

mrouted(8) or the like to find its destination. Or could I use the

route-to option in pf.conf(5) to do this without the complication of

running a multicast routing daemon, something like
	pass in on enc0 route-to ( enc0 $subnet_B ) inet from $subnet_A to

224/4
I'll experiment with that a bit, but any assistance in the mean time

would be appreciated.
Regards,

Damon