ionito:
Therefore, PROGMEM seems to me designed to deal with constant values only. I expected to extend its use, but I believe it is not possible.Any comments?
Not "seems to be", but "is".
PROGMEM stands for Program Memory. It cannot be modified during run-time, unless the code is run out of the boot part of PROGMEM. This is why the bootloader can write to it, and you cannot.