Some topics to discuss: 1. Is there a need for a tag to indicate that we road is invisible yet accessible? Since I think there is, I always use http://wiki.openstreetmap.org/wiki/Key:trail_visibility set to "no".
2. "Vicinalities" a. Atlas sheet A bit technical, but I think we might consider to add a vicinal_ tag like "vicinal_atlas" or even "vicinal_sheet", since "vicinal_ref" is not enough as a referreal. We need to know on which atlas a vicinal road is. These atlasses do not follow the border of the present municipalities, but the borders of the municipalities as they were in 1845. I'd suggest the easiest is to just take the name of the ancient municipality in that case. vicinal_atlas="Drongen" (as a part of Ghent, eg.)
b. Even more technical, sometimes roads in the atlas appear as "chemin particulier". Jurisprudence and Belgian cassation case law is clear (sources available!): these roads need to be considered as vicinal roads. I'd suggest we'd add to the tag vicinal_type a key "particular" or "part". In that case, these roads do not bear an atlas number, nor a name.
Other things might come up.