[AT&T, Rogers HTC One X, Telstra One XL] E:V:A:
Well, I'm waiting on code input from Dan, but without that, there's not much I can do from here at the moment, apart trying to understand the FS better.

Next step we should try to find the RESET_N line, either in/by code or by GPIO if that is used. Which is why someone should undertake the GPIO investigation I proposed earlier and below. This has to be done with great care, and in a local (on phone) terminal app, while NOT being connected with USB port. Because if you by chance trigger the HDMI output (instead), while connected to your USB, you risk ruining something, unless this have already been taken care of in software. If this is still a no-go, we need to start hacking the PMIC to shutdown power rails S4 (LDO regulated) or L5. And if this doesn't do it, I have no idea. Do you have any other ideas/suggestions?

To all: If you're not familiar with writing to GPIO's this is the brief version. I wanted to make a script to give you, but I haven't had time today, so you either wait or do it yourselves. The way I think of doing it, is to set GPIO#n and immediately try to write to a junk file on any writable partition. When this is blocked (or returns error), the eMMC must have been in reset state. I have no idea how fast these things are, so its is very possible that an interpreted bash script doesn't have time to do/see this. But I still don't understand how this would be done, by hoping that WP is be off during eMMC "bootup"? How can you write to it before it's been booted??

Here are the visible GPIO's to check:

[SIZE=2]gpio-3 (TI_AMP_ena ) out lo

"Power Management" lines:
gpio-152 (-- ) out lo 0x05 0x18 0x2a 0x34 0x40 0x58
gpio-153 (TI_AMP_vdd ) out hi 0x0d 0x19 0x2a 0x34 0x40 0x58
gpio-154 (-- ) out hi 0x0d 0x19 0x2a 0x34 0x40 0x58
gpio-157 (-- ) out lo 0x0d 0x18 0x2a 0x3c 0x42 0x58
gpio-161 (-- ) out lo 0x0d 0x1a 0x2a 0x35 0x40 0x58
gpio-163 (-- ) out lo 0x0d 0x18 0x2a 0x3c 0x42 0x58
gpio-165 (-- ) out lo 0x0d 0x18 0x2a 0x3c 0x42 0x58
gpio-167 (-- ) out lo 0x05 0x18 0x2a 0x34 0x42 0x58
gpio-169 (-- ) out lo 0x05 0x18 0x2a 0x38 0x40 0x58
gpio-170 (-- ) out lo 0x05 0x18 0x2a 0x34 0x40 0x58
gpio-175 (-- ) out hi 0x03 0x19 0x2a 0x34 0x40 0x58
gpio-176 (-- ) out hi 0x03 0x19 0x2a 0x34 0x46 0x58
gpio-177 (-- ) out lo 0x03 0x18 0x2a 0x34 0x46 0x58
gpio-178 (-- ) out lo 0x07 0x19 0x2a 0x38 0x40 0x58
gpio-181 (-- ) out lo 0x07 0x18 0x2a 0x38 0x42 0x58
gpio-185 (CDC_RESET ) out hi 0x05 0x19 0x2a 0x38 0x40 0x58
gpio-191 (-- ) out lo 0x05 0x18 0x2a 0x3d 0x40 0x58
[/SIZE]

The way to read/write to these are a little tricky ,and it's important that you first read the current settings, to restore it to what is was before you tried to write to it. Because, like I just said, you may change some internal switches, which could disable your connection or mess up your phones behavior in some other (but usually not dangerous/permanent) way. That's why this testing should be scripted.

Example: To write to GPIO-185:

[SIZE=2]echo 185 > /sys/class/gpio/export
echo out > /sys/class/gpio/gpio185/direction
echo 1 > /sys/class/gpio/gpio185/value
[COLOR=Red]echo 185 > /sys/class/gpio/unexport[/COLOR]
[/SIZE]

Last line should be omitted iff GPIO_n was already visible in FS,

as non-kernel subsystems may need to make use of this, AFAIK.

(iff = "if and only if")

Now, I should warn you that I'm in no way an expert in this, so I can't guaratee that this will work at all, or not have some unwanted consequences on your device. But this is how it works in general on most Linux boxes. So whoever is doing this should make sure he/she is doing it to a development device that can be safely soft-bricked and recovered.

Finally we really have to determine what Qfuses are set, so we can settle once and for all what is the permanent boot configuration. So we know what boot options we can have. If they are not set, we need to physically find out what the BOOT_CONFIG resistors are, unless someone can come up with a better idea how to determine this configuration. (This requires some hardware skill!)