Rule Machine doesn't have list/array/collection-type variables, so this will be pretty difficult. Your best (only?) option would probably be a String variable, but then you run into the problem that String operations are more limited than you'd probably need to be able to select/remove devices:
So, if it were me, I'd second the recommendation above to consider Notifications if it meets your needs. Otherwise, I'd explore that custom app idea more. A
DeviceWrapperList or a
DeviceWrappers (I have no idea how the former is implemented but it seems to do anything I've also asked of
List) is mind-blowingly easy to manipulate or search through with the methods and closures Groovy provides (once you find the right one).