The original version of Triggertrap Mobile allowed you to plug your iOS or Android device directly into your camera to control it via a smartphone app. Version 1.5 of the software now lets you use one smartphone as a Master device, and use it over Wi-Fi to control a Slave smartphone that is physically connected to the camera.
Even better than just separating you from the camera, you can set up as many slave devices as you like — assuming you have enough cameras, cables, and smartphones/tablets to make everything work. And if there’s no local Wi-Fi connection? Then you can use a personal hotspot from your Master device.
While the requirement of one Android or iOS device per camera is definitely a bit of a hindrance, but it is technically compatible with some pretty old devices. You could probably pick up a handful 3rd generation iPod Touches to use on the cheap, which might make things a bit easier. Or you could just round up your friends, and beg them to let you borrow their smartphones for a day.