The problem is that the multicast routing that MT supports doesn't properly handle Bonjour/Zero-conf. I've researched and tried a ton of different ways to do it, and it just never worked. But if you have a working setup that does mDNS service reflection between subnets, please write up a how-to and get it posted on the wiki...people will sing your praises for years to come.
Well, I did a bit of reading and see where the problem is (mDNS using link-local-only multicast group) so obviously routing won't fix the issue. Perhaps some interesting nat/mangle gymnastics are possible....
I think the most available solution for ROS users at the moment is to launch a Metarouter with avahi installed on it and configured as a reflector.
(See the Linux/avahi portion of this link
for an example)
I think an mDNS reflector might be an interesting feature to have if for no other reason than to help people avoid building these ungodly bridged network monstrosities just for IOT support.