How to populate unused flash for an armv7a processor?
I do not think there is an industry standard. However:

Code

Conveniently, all 0xff is an undefined instruction for both arm and thumb encoding (according to objdump).  That means if your program counter happens to wind up in empty flash space, your system will take an exception.  Your exception handler can then reset the system to recover.

Data

Also 0xffffffff conveniently is an odd number.  This means that when dereferenced as word-sized pointer, it will trigger a bus fault.  This exception you can also handle to reset your system.