OK so I got the image mounted (diagrootfs.img in this case) in Ubuntu server and had a poke around.
Here is the output of 'tree' -
https://pastebin.com/yHdEpGhW
There are some 'interesting' files in the root directory:
bin
dev
etc
INTERNAL_FEATURES_ENABLED__DO_NOT_RELEASE
lib
lost+found
mnt
MNTUS_EXEC
opt
PRE_GM_DEBUGGING_FEATURES_ENABLED__REMOVE_AT_GMC
proc
sbin
sys
system
usr
var
I'm not really sure what I'm looking at now...