Air Play-ing music from a sleeping iPhone
If you can avoid bumping the sleep/wake button, you should have hours of playing time using AirPlay. Once the screen is dimmed due to a time-out, you can press sleep/wake to show the current track, just avoid pressing it to force an immediate sleep/screen dim. Let the timer do the work to ensure music playback.

I have Auto-Lock set at 5 minutes (with no passcode lock) and often set down my iPhone on the counter or place it in my pocket while I walk around the house, playing music until a low battery warning pauses playback. If you have to, shorten this setting during the party to avoid uncomfortable musical silences.