You attach the fan to your case and then make a duct out of something (cardboard would do) to your hsf. Then you'd have the added benefit of having nice cool room air on your heatsink instead of that nasty case air.
I thought the exact thing but hesitated to mention it for one good reason, when I mounted the 120mm side case fan and made a cardboard duct and put it over the HSF it actually made the temps rise but making it shorter made it cooler, so if you try this method monitor your temps closely.
I think it would depend on how much of the heatsink you're covering. You obviously wouldn't want to cover much, if any, of it, as then the air doesn't have a way out. To determine the best size for your duct just make it a bit shorter till temps rise a bit. You'd probably end up needing to let it run for a while each time you so this to determine the change in temps. Or you could just stick it to the very top like a fan is and call it good. Whatever you feel like doing.
when I tried it I didn't quite go as far as the fan about 2 cms short of it, just try different legths until you get the coolest reading I found it made about -2 degrees difference for both CPU and System temp