You could never just pitch up, but there were indulgence fares and, yes, you had to sign to guarantee you could afford to get home by commercial means if, for whatever reason, the return half of the trip was cancelled or you were booted off.
(You could also indulge your car back from Cyprus at your own risk. There was the urban legend of the C-130 with a problem where the ramp was dropped and the indulgee car pushed out the back....)