Hello.
I have a similar problem. I've selected 'make paintable' but then the only way I can apply PFX brushes to the (poly) surface is by scribbling over the top of it, and this looks bad.
I also tried making the surface live, then drawing a CV curve down the middle of it, then using 'attach brush to curves' this didn't work for a couple of reasons.
i) the PFX for some reason lay at 90 degrees to the side of the curve, with the grass spreading out in the Y direction either side of the curve.
ii) the stroke just branches out to either side of the initial curve / stroke and does not adhere to the rest of the landscape surface.
Is there not an easy way of 'flooding' selected faces or surfaces with a PFX brush in a similar way to how one does with 3D paint?