Hi, I'm sorry for the necromancy, but actually this thread is positioned at Google as second. And luckily, in my case the mesh has 3.3 million polys. Thomasp script works great. Kudos :)
The "vertex color in OBJ file" is a massive hack that Pixologic added in the last couple of years - it's not actually part of the OBJ file format specification (which is why most importers won't use it). They basically shoved the Polypaint vertex colour information into the OBJ file as comments so "standard" OBJ importers…