Merzouga to Zagora by piste is a very, very long day.
Agreed! In April, 3 of us rode (2 x XT600, 1 x XL600) this in a day via MS6 from Zagora south then east via Rheris/Remlia (yes, sandy and few distinct tracks!) and Merzouga, finally rolling into Erfoud in the dusk, v tired. We had planned to do it in 2 days, but got carried away! Last year, we rode it in the reverse direction, much more slowly and included 2 overnight bivvys. I would certainly recommend doing it with one bivvy to enjoy the varied scenery and terrain.
As a point of practicality, it is far easier using Chris's book, at least to start with, if you do the routes in the direction described. Also, running Olaf, with MO waypoints configured as a route immensely simplifies things. We did this on ETrex Vistas: awesome bits of kit!