Not in a position to test this exactly right now so I am just speculating.
IF this is possible, it should just be a matter of connecting your external display prior to booting, then booting the computer up (I suppose into the recovery partition, or an install disk). Since the computer recognizes the built in display and doesn't know its cracked, it will still display on there (unless you go into clamshell mode by closing the computer after it loads up the external display). I don't know if it will default to mirroring or extending.
Again, that is if it will work. Without an OS installed, I am not sure how it will behave.