Using the code you want then the code "or hidden_resource not_used (orwhatever your non-existant resource is called) should fix the gap.

So if you had a building for Gaul that you don't want them to build in Italy it would be like this (for the first level):

building requires not hidden_resource italia and factions { gauls, } or factions { gauls, } and hidden_resource not_used

this might also work:
building requires factions { gauls, } and not hidden_resource italia or hidden_resource not_used

Also, there is at least one (released) mod which uses the new method to actual effect: Diadochi: Total War. The new method allows a building to have three distinct requirements whereas only two would have been possible under conventional coding.

Strangely though, the building doesn't show up in the building browser except for the first group.