Brush should call some event to update MicroSplat

Can't reproduce: brush seems to be updating MS the same way it does with a splat. See MicroSplatTerrainUpdate scene.

The only thing to mention: make sure you are painting with MicroSplat prototypes - MS creates it's own prototypes for some reason. In terrain inspector, paint tool, Terrain Layers, select splat, click Edit Terrain Layers -> Replace layer. You will see the used layer in an asset list, and it is the one that should be used to paint with Brush.