The description that Ghaban gave of the inscription didn´t hint of any erosion protection. I think that is strange. Sandstone is soft, wind erosion in the desert is strong, only when shielded does an inscription survive. Was the Najran inscription in sandstone or was it basalt? I will check later.
The presence of diacritical points "just like we use today" is in any case remarkable.
The mention of Omar is very fitting, the early date (24 AH) proves the early Meccan pilgrimage, it is one of the few dated graffito... See this article to assess the welcomed importance of the find: http://www.islamic-awareness.org/History/Islam/Inscriptions/kuficsaud.html
By now there is a whole catalogue of inscriptions, any faker would know how to make one fit in. And by accident, it is the dated ones that show a connection with the islamic tradition (Omar and his modesty).
Donner says the sandstone inscriptions are easily made (half an hour). Looks to me an ideal candidate to be forged, especially if no one is applying rigorous scientific criteria to check because well... no graffiti are known te be fakes, ... (dixit Lindstet).
Not saying they are fake, just dont know. I seem to be detecting a graffiti hype coming out of Saudi Arabia, and upto now no critical article examining the veracity in a scientific way.