Ok, i've tried it and it works like a charm.
My Garmin Fenix 3 HR is not listed in the compatible devices list of the Garmin-Specialized App, but that app isn't needed to get the battery info displayed. As long your device has ANT+ (to connect a heart rate chestband) it should work using the Mission Controls "fake-channel".
All you have to do is:
1. activate the "fake-channel" in mission control and set it to e.g. "cadence" (if you don't use a real cadence sensor, otherwise select another channel).
2. go to your garmin (or any other ANT+) device settings >> sensors >> and scan for new sensors while the bike is powered up.
3. for me, it instantly found 2 new sensors "cadence" and "power output". I've activated them both.
4. now you can add the datafield "cadence" to your display and it shows the remaining battery percentage (red circle).
5. you now also can add the datafield "power output" which displays the actual rider power output in watts (blueish circle on the left).
6. with this comes some other datafields like "average power output" (blueish circle on the right) as well as "max power output", "power output per kg weight", etc.
* Sorry for the blurry image, i've shot this while riding on a fireroad
** the remaining battery percentage is labeled "TF" because "Trittfrequenz" is german for "cadence".
