can I take the hard drive out of the macbook, put it in an external usb enclosure and get the hash using terminal with a completely different machine running El Capitain?

extracting the hash will require sudo privilege, while if I don't have the password how can I extract the hash in the first place?

keylogger?

imho you can't do it without elevating your privileges or offline (boot cd)

Correct, boot from external drive with mac and then elevate privileges onto the internal harddrive with terminal, or do it with a meterpreter toolkit...