Cooling radiators in summer?
I think you'd end up running into a whole boat load of issues before you could even begin talking about efficiency. The first and most pressing being the rapid condensation that would occur within seconds of circulating the cold liquid (leaving you with a whole host of other DIY problems you'd need to work out). Which, I imagine would need to be somewhere between 20-60F below the ambient temperature of the air to get any significant cooling effect.

As for efficiency directly, if what I learned in PHYS160 about thermodynamics holds up, it should be exactly as efficient (heat transfer wise that is) as the radiators are at heating the air. But I HIGHLY suggest getting a second opinion from someone more versed in HVAC work, as the closest I've come to directly researching this is looking at data center cooling technology.

**More on datacenter cooling: http://searchdatacenter.techtarget.com/feature/Modern-data-center-cooling-systems-No-muss-no-fuss-no-waste