AFAIK, it looks like RouterOS just copies the mDNS UDP packets between the selected interface.
It’s roughly same approach as http://forum.mikrotik.com/t/mdns-between-vlans-with-just-bridge-filters-look-mum-no-containers/173295/1 Just implemented at a low-level.
Basically RouterOS will look for mDNS if configured in /ip/dns, then re-broadcast any mDNS the router gets on ANY of the mDNS interface to ALL of the configuration mDNS interfaces.
So it’s not some flexible mDNS “router”, at this point, it just copies/mirror/repeats everything bi-directionally. The reason why this get more complex is if you want to define “one-way” repeating (i.e. mDNS flows uni-directionally). Right now, basically, you can create ONE mDNS discovery zone, that “shares” everything between the interfaces configured in /ip/dns. Perhaps more is planned, but I think that’s what it does in beta3.