1 pointSwitch your Velocity Type to Volume in the Collisions tab on the FLIP solver. You'll need to increase your substeps too.
1 pointFrom the help card. You can then write a vex expression to override the force scale parameter per particle.
1 pointAh I fixed that issue, I had to use an absolute on the flat vector in the displace vopsop. I think this method is quite simple now? It may be oversimplified as I haven't tried on many examples or curved surfaces, uvs etc. But I would be interested to see what people think, and if i'm missing something obvious??... Here's the slightly newer version: fracture_detail_v5.hip