Hello,
Being a newbie I hope I do not step on anyone's toes by saying this but I would have concerns about the willingness of people to download an app for the sole purpose of being able to change songs on someone's light show only when they're in range of the WIFI from the house... Personally, I wouldn't waste my time with downloading the app, especially if it had GPS tracking enabled...
My recommendation is to build a Access Point(AP) which has captive portal capabilities. The captive portal will automatically redirect their browsers (of any device connecting to the AP) to a address of your choosing. This approach is nice from a security stand point because you would be segmenting the people connecting to the open AP from your homes LAN/wirless traffic. You can also lock down which ports on the xLights host they will have access to since the AP is now a firewall device and it would greatly reduce the risk of attacks by having an open AP.
Being that xLights is a cross platform app, I would recommend using something that is cross platform as well to host the web pages from. I know there is Apache(2) for windows, Linux and I think it is even installed but disabled in Mac OSX by default.
Getting back to setting up the AP w/captive portal. I have built dozens of APs w/captive portals for offices & restaurants and a lot of them were nothing more than discarded PCs I had laying around after building someone a new computer. All I did was put a wireless card or USB dongle in the PC and install something like pfSense, OpenWRT or monowall on the computer then some configuration. Here is a link to more information about captive portals and a list of some popular AP/captive portal software: You are not allowed to view links.
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Ryan