There was some chips with programmable registers - these registers define from which adresses writing is enabled.
This is OK for protection against firmware failure, but for
human intervention....

You want something as "black box".
Black box can be shielded and sealed.
Interface to box - serial.
In box will be miccontroller - part of flash can be used
as memory for event storing and this is best solution . Other variant is to use additional memory chip. For better security
both variants can be combined.
regards