If someone is interested, I developed for a customer a bootloader that reads a file from an SD card and reflashes the application.
The code is based on FAT16 and SDCard libraries for Arduino, even if our hardware is no (longer) Arduino.
It works with 1280 and 2560. The bootloader area should be configured at the maximum size.
It wasn't difficult to develop, and I suppose everyone can do the same in short time - but this is already working and tested, so it can be a starting point.
Let me know...