I´m not sure if it is impossible.
But in the rogue circuits case, technically the bootloader is writting the code, the other acessory MCU implements most of the FAT32 and allows the bootloader to write itself using UART comunication between the 2 MCUs. It is mandatory that you burn a special bootloader to use this approach.
In the case of an arduino programing another arduino is the same case because technically is the bootloader that is fetching the code from the other arduino that is acting as a programmer...
Can you elaborate on the reason why, one MCU cannot write the all flash of another MCU Mike?
regards