I received lots of firmware dumps that allowed me to do some investigations. Thanks to everyone for the files. First of all, beside the regular AD-5280S and AD-7280S bootcodes, there are also the AD-5280S-CB, AD-5280S-CB for duplicators (1.Z* firmware) and (seems to be) AD-7280S-CB bootcodes.
Comparing the regular with the CB bootcodes, it was impossible for me to find out whether they are really compatible or not. They might be, but flashing the wrong bootcode may as well brick your drive. Usually you should still be able to flash back to the original bootcode however.
I sent bootcode flashers along with firmwares to Lightning UK! - not sure if he’s brave enough to do some testing.
I will provide updates as soon as I have them.