I'm not sure if anyone in the R-Pi design department reads this forum, but I just noticed a different approach to USB power is taken by the Olimex "OlinuXino Micro" which is a small ARM board. It is actually advertised as a "Raspberry Pi alternative" although it is lower-spec in many areas (details here: https://www.olimex.com/dev/imx233-olinuxino-micro.html ).
The Olimex design uses a Silergy SY6280 protection device on their USB host port +5V supply. This is a programmable current limiter, also with a shutdown input and reverse-current protection (no back-feeding from cheap powered hubs). The FET on-resistance is a low 0.08 ohms. The output current limit can be set from 0.4 A to 2 A. The part comes in a small SOT-23-5 package, and is presumably cheap. I'd never heard of this part before... the documentation (if any?) is apparently only in Chinese, and it seems the part is not distributed in the US.
The OlinuXino Micro is open hardware. Here is the board schematic:
