I've run a belt on my Norton Commando for the last six years with the primary cover on and never had any heat problems. There's really very little to cool the internal components even when you run a chain in there. Of course you run oil with a chain, but it's more for chain lubrication than it is for cooling.
If you're concerned you can either put some very small spacers between the cover and the crankcase or you can cut just a few inconspicuous vent holes. I used to know a guy who raced a Triumph with a belt drive and used a stock primary cover. He did have two small holes drilled at the front and two at the back. I asked him if he ever had problems with heat and he indicated that he'd not. I doubt that you will ever heat a clutch on the street the way a clutch on the track is heated.