hi
this track definitely exists—I did it from Majunga to Morondava in 2015 in a 4x4, and you can do it by motorbike too.
For both modes of transport, only attempt it from July/August to early November. There are two river crossings, one of which is pretty tough.
If you're in a 4x4, do it north to south.
For cycling, you’ll need to be completely self-sufficient for sleeping—there are stretches of over 100 km where you won’t see a soul!! Water is essential!
From Morondava to Morombe, no problem, but plan for the same dates for the river crossings!!
Also, from Manja to Morombe, the roads are being completely redone.
Soon there’ll be a bridge over the Mangoky at Bevoay.
Of course, only travel during the day, as always!!
For the river crossings, don’t trust the young guys hanging around who’ll promise you a safe passage—you’ll end up stuck or risk drowning your vehicle. Instead, be upfront and offer "vola" (money) if things go well, and negotiate!
happy travels
jipi