Would just unscrew the fans from the rad, hold it inside the PC case and spray it, then re-mount. Wouldn't involve taking the CPU block off or anything overly complicated.
While in effect you will have just transferred the dust to the rest of the system, its easier to clean there.
The tubing that connects the CPU block to the radiator is very stiff and I fear of dropping it and breaking other components, as I wouldn't be able to replace them. But when I do blow compressed air through the radiator, no dust comes out at all, but I can see a layer of dust over the fins of the radiator, but I can also see straight through the radiator with no blockage. Does that mean it is clean? The temps I get go no higher than 45-50C when playing very intensive games (overclocks to 4.0Ghz) and it idles at 28-35C (which I think is irrelevant as my CPU down-clocks to 2.6Ghz when in idle)