I wonder about the need for an Air Gap.
This would presume that the walls of the Freezer are a major source for Radiating Heat. From the way I see my Freezer working (sadly just for food... not beer yet...) the temperature of the outside walls does not change. If it was radiating heat, it would likely be significantly colder than the ambient air.
The fact that the walls and lid of the freezer are so well insulated would tell me that the manufacturer does not want to radiate heat from the walls... they want the walls to Keep the Heat In. So not having an Air gap might actually improve the insulation values of the Walls.
You would still want to ensure that any Ventilation points are free and clear to allow a good exchange of air over the chilling coils or what ever method of cooling is used.
(Edit) Also I assume that a Keezer would be run significantly warmer than it would be normally when used to Freeze Food Items. We don't want Solid Kegs after all, so that would imply that the Compressor is not working as hard as it would under standard circumstances, so it likely could handle any extra stress a Skin might add.
I am no Cooling Expert, so I am open to having my assumptions changed by those who are experts.