I just did a test run with the wacom board: I was able to inject 5v into it ! yay!
As I suspected, the injection point is right out of the voltage regulator (left side of IC102, near C106, I hooked myself directly onto the regulator leg). I also computed how much milliamp it was pulling on the 5v: ~97.5mA -- we can say 100mA. Which seems to be within USB limits.
BTW, somehow the mA reading was also 100mA when connected through the normal 9v volt input power (same for 12v!) (the input power can be anything from 9v to 12v -- and it is true, below 9v it starts failing). That means the higher the voltage, the more "power" (watt) it is pulling. So feeding 5v is more economic than feeding 12v or 9v.
Now my test is not fully completed because I had no PC connected to it -- I only had the little led on the board to tell me that it is functioning and accepting the pen.
I quickly looked at the RS-232 on the board -- and *I think I was wrong*: it seems to use the 5v supply and not the 9-12v (!) -- The next thing to test would be to connect a PC to it (install drivers, blah, blah blah

) and check if the serial port is functioning correctly with the 5v supply. That also means that the sparkplug FTDI breakout board might not be required (neither the hack to find the proper signals to bypass the serial port driver) and a standard usb-serial converted could work. On the other hand, the FTDI breakout board solution might prove better in terms of energy efficiency (we would bypass/remove another chip from the equation). Of course, most of the power is going to the LCD backlight, but hey, it is good to note our options here.
Note: If this setup is to be made permanent, I would remove the regulator or at least lift/cut its left leg -- so we disconnect everything on that side of the circuit -- not sure how the regulator likes it in the long run to be feed power from its output while nothing is coming in its input. Probably not that good.
Note2: when injecting the 5v it would be better to leave the main switch in the OFF position -- that isolates more of the circuits.

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