The NFPA frowns on extensions cords. Those have so many hazards. But you know about that already. Still, there are such things as GFCI extension cords.
One risk you might encounter if you install a light fixture that does NOT have a receptacle is that you force the homeowner into using an extension cord (or so it may be argued against you in court). Unless you can find a light fixture that has a convenience outlet ... and with GFCI built in ... you might not want to replace it as that effectively removes something they already have, creating a greater hazard.
