The point of a guide number is to specify the relation between aperture number and distance. For zoomable flashes there are still extensive tables considering ISO number and zoom length. The relation with ISO is straightforward (the square root of the ISO number factors into the guide number). The relation with zoom length tends to be inscrutable. At wider zoom settings, zoom length and light angle are not fully inverse (should be 2 arctan (18mm/f) horizontally and 2 arctan (12mm/f) vertically with f being the 35mm equivalent). But at least at longer zoom levels, one would imagine a somewhat predictable relationship.
Assuming that the zoom head makes best use of the available light output (and for the sake of simplicity we are zoomed far enough that arctan(x)=x is a reasonable approximation) what would be the expected effect on the guide number? Whatever I come up with seems so far from the actual tables that I seriously doubt my assumptions.