I added a custom queue feature last year that allowed me to add a "remote control" so viewers can select the songs right from their phones. This basically does what the op is looking for. It stops the running schedule, plays the requested song(s) and then returns to the regular schedule. It doesn't pause and resume a running sequence though...it stops it, plays a requested song, and then restarts it.
My show runs a random/repeating playlist. When user requests a song xlights stops, plays the song and then starts the random playlist again when finished. The changes are rudimentary at this point but it worked great for me this season.
Let me know if you're interested and I'll send you the modified xlights program that you can play around with.