Just for completeness: the right way to do this is to add the json under ~/.platformio/boards and the other files as described in https://community.platformio.org/t/how-use-a-same-chip-but-with-different-environment/18082/4
IT WORKS
I got a first working version, using an ATmega1284P running on MightyCore 2.2.2 (as soon as I began developement, they switched from Optiboot to Urboot in version 3.0.0, whicht does not include the copy_flash_pages function anymore, but I'm working on a work around there).
Only thing I could not solve is to do a CRC check of the image after download...
You can try it out here: https://github.com/eiten/MySensors/tree/FOTAInternalFlashTest
It's not beautiful yet with much Serial.prints in it, but maybe it helps you if you find errors.
