The only problem is if your radiator is in a really good spot, chances are you could be heating the block with the HSF added on there.
Water = .6W/(m*K)
Air = .025W/(m*K)
.6/.025 = 24X
So Water has 24X the thermal conductivity of Air.
Air = 0.001297 J cm-3 K-1
Water = 4.184 J cm-3 K-1
4.184/.001297 = 3,225
So water has over 3,000 times the specific heat capacity (ability to store energy/heat).
Bottom line, water is incredibly better at transferring and removing heat than heatpipes. Heatpipes are much better at transferring heat than solid metals, but not better than water. To incorporate a design to include heatpipes, you would need a really thick base, and that thick base would hurt the ability of a water block to quickly transfer heat to the water.
Some thickness is good to distribute heat, but you'll find that most waterblocks are somewhere between .050" and .125" in thickness and that's because water is really really good at transferring heat and storing it.
When I first started water cooling I always thought why someone doesn't make a huge tower like waterblock with a huge surface area. The reason is the thermal property difference between water and air, they are just soo different.
My 2c anyhow...
You could always try it. I've had a few strange ideas I tried...always fun to give it a try even if it doesn't pan out..