I mean to clear the *whole* memory upon startup. I'd even consider that harmful.
a) why do you consider that 'harmful' the RAM is random after power up.
b) if a problem, then 'misuse' startup

The traditional way is to clear (write zero) all static (i.e. global and local "tagged" as static) variables which are not explicitly initialised, and initialise the rest.
'Tagged' is DEFINITELY not "The traditional way"
Mr C hater: in assembler, would you carefully clear only what was not to be initialized or would you clear the whole shebang with a simple loop and then set the initialized values

Btw., how does Keil know how much XRAM is connected?
as posted
The startup code (if not misused) i.e. the correct value is entered. an incorrect value would constitute 'misuse'

Erik