I have experimented with DD cooling as well and agree that Athlons just dont have enough surface area for water to cool.
I cooled a P1 233@292 with DD made from a plastic garden chair foot with an inlet through the centre. Got better than WB results but could not get similar results with an Athlon.
How about making a tiny copper WB with a base the same area as the CPU top surface; Thin pins about 1cm long and a very thin base. Attach it with a non soluble TIM and perhaps a spring loaded centering mechanism to hold it down in place. I know you no longer really have DD cooling but you still get to DD cool the sides of the processor and increase the surface area without large delt T's from a thick base.
Just a thought.