As the irishman said when asked for directions: "Well, I wouldn't start from there..."
As the guys have said, the dunes west of Mhamid are a nightmare on big bikes, as is the sandy piste by the Chegaga dunes. I accompanied a couple of very experienced riders on 1150GS's through there a while ago and the soft sand and heat damn near killed them and their bikes. That was in April.
If you're going to do it, you'd be far better starting from Tagounite.
Get on the piste heading NW from the centre of town, following the telegraph poles, keeping the ridge on your left. In about 17km there's a gap in the ridge so you can turn SSW. As you come onto the plain, you'll pass through an old abandoned defensive line.
In 10km or so, swing SW - you'll pass a well in a few km. Continue on this heading and you'll pick up the main piste coming in from Mhamid on your left, thus avoiding the dune field. Carry on due W towards the Oasis Sacree. The piste is a bit rocky for 20km but eventually it joins up with the piste coming in L from the Chegaga dunes. You'll be ready for an ice cold Fanta at the Titanic Cafe.
With a sugary drink inside you, it's time for a blast across Lac Iriki. The lads at the cafe will tell you whether or not the lake is dry. If it's wet, take the very rocky 'piste de la montagne" to the north of the lake. It'll take you all the way to FZ but it's bloody hard work.
If it's dry, continue across the lake WSW towards a large mesa. Just before you get there the track splits. NW is the rocky track to FZ. If you head SSE, you can follow the old Paris-Dakar piste which is clearly marked with balises (large piles of rock). Be aware that the first 200m is soft sand, after that it's OK.Keep the saw-toothed mountain range on your R. In 9km, there's a military checkpoint and the track swings due W.
In 35km you'll come to a track junction. Turn R heading due N for FZ. There's a checkpoint after 10km then continue N across the plain for 25km to join the N12. In 8km, you'll be tucking into 'brochettes de poulet' at Rachid's cafe in the square. He's probably got a couple of

s in the fridge.
Temperature is going to be the big limiter, it's still mid 30s in Zagora at the moment. You'll need to keep to the good firm pistes and avoid as much of the slow, strength-sapping rocky stuff as possible. Why not head out from Tagounite and see how you get on for a few hours? If it's too much you can back track to Zagora.
Happy trails,
Peter