Ok, I finally got around to looking at this. I had luck by doing the following on OSX 10.10.4:
0) Go to Finder
1) Hold all three buttons at once (command + shift + G)
2) Enter "/System/Library/LaunchDaemons/com.apple.discoveryd.plist" into the text field
3) Press enter (which hopefully opens that file in a text editor)
4) In the file look for the section "<key>ProgramArguments</key>"
5) Remove the part that says "<string>--no-multicast</string>"
6) Save and close the file
6) Open Terminal
7) Run "sudo launchctl unload /System/Library/LaunchDaemons/com.apple.discoveryd.plist"
8) Then run "sudo launchctl load /System/Library/LaunchDaemons/com.apple.discoveryd.plist"
Hopefully now when you enter the local url of a given edison in the browser (e.g. edison.local) it will resolve to the IP address of that edison. Otherwise it's a bit tricky to figure out how to shell into the edison, particularly if you don't want to usb tether it to your machine.
Hope that helps somebody!