I found your question very interesting because you want an antibody that identifies neuronal progenitors, but not glia. Neuronal progenitors are glia. I wonder what you expect this antibody to mark. Can you explain it to me? Just curiosity.
As you said, neural progenitors express GFAP and in the adult brain can be found as astrocytes in some areas (SVZ, Hp and Olf). However, a more differentiated state of the neural progenitor can lose this phenotype. An immature neuroblast can express DCX and before Nestin. But, I think, what he is asking for is a marker that could be expressed in a stage more or less like a Nestin-expression one but it is not found in the astrocytes.