I've tried to accomplish this as well.
In the point density texture there's an option to source the values from vertex weights. Unfortunately this only seems to work with volumes, as any output that comes from it when used as a surface texture doesn't seem to correspond with the mesh weights.
There's a workaround though, but you have to set your dynamic paint to texture and bake it out, that way you can load it in to Cycles as an image sequence. If you edit the weights you have to re-bake it, so not the most elegant of solutions.
Maybe someone else has a technique that's more efficient?
edit: workaround tutorial