Well I found a way to do the build in two parts:
buildiso -x -p xfce
Make tweaks in /var/lib/manjaro-tools/buildiso/xfce/x86_64/
buildiso -p xfce -zc
That enabled me to solve some other problems but if I hack /etc/passwd the ISO when booted up requires a password for user manjaro. Entering "manjaro" didn't work

So I guess I'll live with a manual chsh for the moment.
In the process of doing all this, I've also managed to integrate several AUR packages successfully so I'm a pretty happy camper overall!
Thanks for all your help guys and have a great weekend!