Is this possible - has anyone done it?

Sure, it's possible, but I wouldn't recommend it because you won't be able to get far enough into the country to see anything of real value. You'd probably end up in Tangiers, which IMO is a pretty scummy place. To appreciate and enjoy Morocco you need to get to Fez, Casablanca, Marrakesh, or even farther south. A day in Tangiers might well turn you off North Africa, which would be a shame as it's such a magical place.

Gibraltar might be the place then to tack on after madrid, seville, and before Malaga and Granada. I don't want a tijuana type experience and that is what Tangiers sounds like.

I haven't been to Gibraltar, so can't comment, but "a Tijuana experience" is exactly what I would call a trip to just Tangiers.

Hi -
I'd just like to confirm what the other posters have said above. Tangiers is not where you'd like to spend a day to say you've been in Morocco! Been there twice as I was coming and going and hopefully won't be back again!