I am still yet to see somebody implement off power read/write of the eMMC chip. ATF manages with so many wires by the fact that the chip
is still wired to the CPU and all control signals are handled correct. If you watch the protocol on screen you will see that ATF FGPA asserts
low's and high's(high levels slightly higher than phone's 1s and lows via bridging few gates in parallel to assert low impedance on the bus) during
the important boot mode check when the eMMC is in 1bit SDIO mode, immediately after restart.
To use external MMC reader and not powered phone PCB you will need bit more lines connected, maybe less with a small microcontroller
development board and using the HW SDIO interface(most chips have it these days) e. g. - STM32F4Discovery, but you still need to cook
custom boot sequence yourself.
The standard firmware in off the shelf reader would probably not manage to take control of the eMMC as i suspect it not communicate in 1 bit
SDIO mode, probably do in the beginning and then switches to wider interface.