Fine for cars, but someone else picking up your car might be a problem
We did leave a car at the airport car park while my partner (the car owner) had to fly home for a few days. We found the parking at the airport to be fine and there were plenty of other people also leaving their cars there for long periods of time.
But I think you might have difficulty if you want someone else to pick the car up. When you hand in your car keys and car documentation at Marrakesh Airport, they don't give you a receipt. They register your keys and documents under your passport number and CIN number. When you return to Marrakesh, they check your passport details to hand back the correct documents and car keys to you. So I should think that if someone else showed up, even with a letter of authority from you, customs would not hand over the keys and documents.
The other point is that this process can only be done by the person who brought the car into Morocco in the first place. I stayed in Marrakesh during the time my partner came home, but we still had to leave the car keys and documents at the airport because he is the owner of the car and we brought it into Morocco in his name.
The reason for all this is that if you bring a car into Morocco, you also have to leave with it. This is to stop you selling the car in Morocco and means you are legally responsible for the car while it is in Morocco.
We found the customs people at the airport to be very helpful. If you are in Marrakesh, then I would suggest going to the airport to see them and talk it through with them. There may be a way for someone else to pick up your car, which they would be able to advise you of.
I don't know if I have really answered your question here, but I hope it is useful.