Yes, most likely the bootloader is different (the chipset supports encryption and signing from bootblock level up) - I didn't check myself, but the bin/NK disassembly instructions are still around (they are pretty generic). Worst case would require the memory to be re-written externally (by programmer) as the built-in-flasher could fail to update/change certain flash-areas.
Please note that I'm NOT an expert in firmware bootloading etc. but more a systems generalist.
The described method with "force update" above could really be the key to un-bricking the camera.
Basically any step involved in bootup has to work - otherwise the camera will seem bricked:
- bootloader
- read & parse configuration data
- boot applications
For example: if one changes the bootloader and the binaries, then one also has to change the config files as the old software won't be able to expect - or parse new checksums or obfuscation layers introduced in later versions.
A complete copy of firmware AND configuration could be a method of downgrade - assuming one does not forget the serial number inside I²C.
Another approach would be to patch the code that reads the serial from I²C to return something pre-defined...
Just an idea for anybody who wants to try