Yea a proper ingame clock would be really nice to organize events etc.
As for battery indicator, there’s countless headsets around and most have no battery! Controller batteries are rather the issue. The rift gives out a warning about these, I guess you’re on a quest, it’s odd there is none, however, you just need to press one button to see the status on Oculus Home.
So I don’t think this feature is doable or makes sense to be done by an app. This is just what every vr user just has to get accustomed to, even a warning would not help much in a dungeon, it will interrupt gameplay this or that way, since it only comes minutes before battery is off. So always make sure it is charged before starting a run or set an alarm once you know how long you can play with a fully charged headset, approximately.