You can do that by taking a mix/aux out, sending it to battery powered head phone amp and controlling the mix with the monitor mix app like QU You or SQ4 You.
That works (and works well), but does require that the system have enough available busses to support it. The beauty of the ME system is that it doesn’t count against your buss counts. I can definitely see where a ME “headphone amp only” device would be useful for this reason.
The technical requirements of this however are unknown. As far as I know, there is not currently any way to control your ME mixes via an app and it’s very possible that the ME system doesn’t provide a way of accomplishing this (through an app). Without app support, a headphone amp only device would never work.