A custom Guide Mask for the Clump Hair Curves node group should do the trick. This can be either a vertex group that you can specify in the modifier interface directly or an image texture mask.
In the example, the quick fur of Blender 3.6 was used and the Clump Hair Curves asset was added.
Option 1: Vertex Group - The vertex group must be on the surface object ("Curved Plane"). The weights require vertices (e.g. a subdivided plane ;-) and the same options as described below.
Option 2: Image Texture Mask -
Because you cannot specify the image texture in the Geometry Nodes modifier directly you need to move it to the nodes editor. This is done with the dropdown menu entry "Move to Nodes" of the modifier.
It creates a wrapper GN node tree with the Clump Hair Curves node group in it. In the Geometry Nodes editor you can now define the options.
Clump Hair Curves Options
- uncheck Existing Guide Map so any existing guide map is not used (Interpolate Hair Curves creates usually one)
- define a Guide Distance value for the creation of the new curve guides
- connect an Image Texture node to the group's Guide Mask input
Last but not least, you need to
- create a new group input for the modifier so you can specify the UV map of the hair curves' surface object ("Curved Plane" in the example). This is needed to map the image mask properly.
(To create the input just drag the Vector slot of the Image Texture node to the empty Group Input node slot and rename it in the side panel of the editor (N).
- define the UV map in the modifier settings ("UVMap" for the default UV map)
requires Blender 3.5