After googling, I still haven’t found a way to scroll up the output of the GRUB 2 shell. However, there's a way to pause output (like the behaviour of the more command) - the
pager environment variable.
If set to ‘1’, pause output after each screenful and wait for keyboard input. The default is not to pause output.
So, issue a
set pager=1 command first in the GRUB 2 shell, and then the output will behave like more.