I have tested two Y-cables Xiangfu bought:
They both have the ID pin connected to GND in USB Micro B port as
the OTG host.
The first one should meet all the needs:
Works fine when connecting usb keyboard/mouse/disk on the OTG slave
(the standard USB A female)
The power branch does deliver power to both slave and host, only
failing in charging a Nexus5. But it can charge a Nexus4 and power
up a RaspberryPi.
One problem is that the power branch has a Micro B female and is not
long enough. But we can connect it with a Micro B male to Standard A
male cable which is very easy to get:
The second one has a chip inside.
It cannot work properly when all three ends are in use.
Another cable Xiangfu ordered should arrive in a few days. I will
test it later.