You can’t, really. Maybe you can forward the multicast announcements to another subnet.
I see reasons to segregate a WLAN from a wired LAN, but I wouldnt account for it, if its a private network. *shrug*
Or enable multicast routing, if your router supports it.
Failing that, you can run a proxy responder on the remote subnet. There are several mdns proxies around — I use network beacon, but check the wiki entry on ssh tunneling. Ignore the stuff on ssh, just see the part about setting up a proxy mdns responder.