all you need really is a shader than can control whats displayed using a hard falloff curve, and the overlapping of the gradients creates the meta-ball effect
hmm I may have done this already a while back. An alpha remap effect. Note that since we're using premultiplied alpha, transparent colors will lose precision when made solid. But it works, I used it to make clouds that slowly vanish.
with metaballs, you'd just put alpha-ramp spheres in a layer, then remap the layer's alpha.
I'll look for the file, not sure if I uploaded it here.