Re-resurrecting this thread, I had a bricked CMU after some reckless installation/updates of tweaks (one of the problems with being on the bleeding edge) in preparation for CarPlay. I didn't need to resort to any serial connections but just (sic) flash the EEPROM/Flash/PROM.
I was stuck in a boot-loop - actually stuck at the Mazda logo after upgrading to V70 firmaware.
The information, here, really helped. If you are stuck in a boot-loop/lock, and even replacing the tweak files to the original (which I did to no avail) this is your second to last resort. The last resort is about $1000 on a new/replacement CMU (used ones in an unknown state may be had for cheaper).
1. Purchase both the CH341A programmer board and the PROM test clip as linked in the original post;
2. Download the CH341A drivers and programming tool;
3. Make sure you have the .UP files on a USB drive;
4. Dissassemble the dash as if replacing the USB hub (see the android/carplay post here
4a. You do not need to take apart the lower console to get access to the CMU;
5. With the CMU/screen out, you can disconnect the two connectors to the LCD screen, and unbolt (3 screws) the screen (removing the screen may not be necessary, but you may not want to handle it too much, to avoid potential marking or damage), and 2 screws for the rear bracket;
6. Remove the bottom panel of the CMU using a torx wrench (T20);
7. Connect the test clip to the PROM chip, clearly visible (*);
8. Connect the other end of the test clip to the CH341A USB board (*);
9. Plug the USB board into your computer (I used Windows 10 Professional);
10. Read the PROM contents, and save them as a .bin file;
11. Verify the PROM with what has been read;
12. change byte at offset 0x0010000 from FF to 00;
13. Optionally, save this 'new' file;
14. Write the contents to the PROM - this takes longer (about 5 minutes or so) to write than read;
15. Disconnect from the PROM;
16. Take the CMU back to the car, reassemble screen onto it, and connect back to car;
17. Turning the car on will/should show an 'update failure' on the screen, and wait for you to insert the USB with the update files;
18. Insert the USB, it should autodetect the .up files and go into the standard firmware update (about 20-30 minutes).
(*) Using the PROMmer:
Look carefully at the CH341A software and the image in the original post: the left most pins of the ZIF (Zero Insertion Force) socket are used, not the full 16 pins. The cable (or the CH341A, I can't recall) come with an additional board with a 16 to 8 pin converter. The pin layout on that board converts, correctly, the 16 pins on the PROM and the 8 pins required for programming.
If you get it wrong (orientation, etc.) it simply doesn't work, but doesn't damage anything. The 'detect' button in the CH341A programming software should show the identified MXIC chip - but the wrong model. The correct chip - as shown in the original post - is the MX25L6445E
(it's printed on the PROM).
(I spent a lot of time trying to get this lot to work - eventually found that the ZIF socket on my CH341A board was not soldered correctly, and had to frankenstein something together).
As far as obtaining the CH341A programming software, you might need to hit up some dubious Russian/Chinese web sites. It took me a while to find the software which both worked and seemed to be the least 'unreputable' site. It needs to look like the image in the OP, but there are a couple of other applications which may work.
As a final note, this whole experience can be quite nerve wracking - just remember, at this point, you would only do this with a bricked CMU with no other avenue of recovery, so have very little to loose (except about a grand on a new CMU).
Also note that I am, actually, an engineer with the motto "If it ain't broke, you haven't f****d with it enough".