Here are some threads I remember participating in where something like this would have made writing the Rule in question easy or possible. In many cases, the new Rule 5.1 "last event device" feature would address the need:
For cases like that last one, I normally end up writing a custom app. Here's something I replaced with a custom app that I wouldn't have needed to if something like that existed, though that request is more than simply a device variable (but rather a collection of devices, and the ability to do things like store matching ones to that collection--apparently a webCoRE feature people ask about sometimes):
