Wonder if any OpenWRT/LEDE supported access points provide full power output on their USB interfaces.
The goal is to power a Raspberry pi 3 B+ or Raspberry Pi Zero W using the USB port on Access Point. Thanks!
LOL
Seriously, no way.
RPis are picky and need a beefy power supply. Relying on the USB power provided by something else than a dedicated USB power supply calls for issues. Using standard off-the-shelf USB cables who's performance under load you havn't measured yourself calls for issues.
Sure, it will work some time, but at some point in time you will wonder about unexplainable filesystem corruptions and other strange, unexpected behaviour...
You can save yourself a lot of time and headaches if you use RPis only with a suitable power supply and only with suitable USB cables who's performance you have measured yourself under load.
@tmomas You might have saved me lot of pain. Thanks! 
You're welcome! 
As a sidenote on the importance of USB cables and measuring instead of believing:
From my own notes and experiences with RPi2B:
One would think that 0,5m / 0,55m are better than 0,6m, but that's obviously not the case (see above). -> Only believe what you have measured yourself.
Regarding power supplies (my own measurements):
-> I'd recommend Goobay 71889. Comes with fixed micro usb cable and has output voltage always >/=5,0V
IKEA Koppla showed also good performance at only 0,1V lower output voltage than the Goobay.
Useful tools for measuring performance of power supply + cable under load:
- Electronic load like this (you will find many more for small money on aliexpress):
- USB tester. Keywords to search for on aliexpress "USB comprehensive tester oled"
You do want to have the all-inclusive version ("comprehensive") with 4 digits after the comma. Instructions included in the box.
-> Use only power supply / cable combination which delivers >4,50V @2A, the more the better.
According to this table a zero-w doesn't draw more than 250mA. I think any USB host port should be able to provide that.
Don't think, but measure. Everything else calls for problems, even if the zero-w consumes only 250mA nominally. It's the spikes that create the problems. Apart from the power the zero-w draws, keep in mind that your "power supply" also consumes power, and you do not have any clue when and how much power your access points will draw.