[QUOTE=hawhill;2128834... Currently I'm really missing a simple test point toggled by a GPIO, preferably, to have a simple test for uploading and running my own code in USB ROM kernel (i.e. SE Ringo) mode. Again: ATK will *not* work as it is for writing/reading data. The RAM kernel it is equipped with is designed for their own evaluation boards - different enough compared with the Kindle.[/QUOTE]What I did when developing a custom uboot for MfgTool was to have it print to the serial port, so I could see the results while I poked around in it.
In my case, my K4 somehow corrupted itself, writing garbage on all the idme vars (including Serial number, PCB serial, etc.). Without PCBSN it did not know how to initialize RAM and just displayed an error message to serial output and halted. My custom code forced (and wrote) all the idme vars to values from a backup copy (inside a diags XML output dump). So the custom MfgTool profiles began life debricking my SEVERELY bricked K4.
Instead of toggling a GPIO you could test your code with serial output. Although, if we can find a few GPIO pins we can add a bit-bashed SD-CARD interface.