Thanks for the replys.
1) I enabled the loopback checkbox, but that doesn't seem to help; I've included a screenshot of my NAT settings.
edit: I'll take that back, it does work. For some reason my desktop PC hasn't woken up to the reality yet, but my phone connects succesfully.
When you talk about DNS, where and how exactly would I configure this? Would this have to be set up on the connecting device or is it possible to tell USG to do this? I don't think I can add custom DNS values to unrooted Android devices.
2) I've included a screenshot from Wireshark showing the relevant lines. I captured interfaces WLAN & LAN. 192.168.100.6 is the Squeezebox and 192.168.100.4 is the server. These lines just keep repeating. I guess the conclusion is that no magic packet is being sent because the Squeezebox receives no answer to the ARP request?
On the bottom is an example of my desktop PC asking USGs MAC address and USG responding.
I'm really no expert with these things, so if you require more information just tell me and I'll try to provide.