IMO the best riding - good, empty roads and interesting landscapes - is down south, but September will still be pretty hot, and away from the coast, riding slowly on rough tracks will feel hotter still. Carry camelbaks and use High 5 Zero tablets (or salt + sugar). Makes a real difference I find. Ten days in Mk might give you 5 or six down south.
Many riders' off road ambitions exceed their actual ability, especially on big bikes, but of course there's only one way to find that out ;-)
Jebel Sarhro (MH4) north to south is a classic route and a good start: great scenery, rocky but downhill mostly so doable on a big bike. Just sit back and let the suspension do its thing. I would steer clear of any of the Alg border pistes which have sandy sections - not everyone in your group may get the knack. It's no joke biting off more than you can chew and getting stuck down there in 35°C+.
Ouarzazate to Foum Zguid (via Taz) is a brilliant road ride giving a good feel of descending into the desert (20+ kms gravel/repairs before FZ will give you some sport). Come back north via Akka Ighern (easy piste or border road) up to Taliouine and back over the Atlas via the Test pass for Marrakech - all great riding.
I'd also hotel rather than camp. End of the day you'll be hot, sweaty and tired and in Morocco I find its hard to camp wild and not be visited. Sharing a room half-board from 15 quid pp - and there are many more hotels down south than tourists to fill them.
I would also consider dumping the baggage at a hotel and doing day pistes. Less weight (esp. if you still bring camping gear), more fun and less damage if you fall over. I am sure others will chip in with ideas.