I think I see a way to do what you're trying to do. You might have to dedicate a layer to your bump map, but you'll be able to edit it locally.
Create a pixel layer, give it a solid color (I'm using a darkish green, #1d710c. Call it 'grass'.
Create another pixel layer on top of that. Call it 'bumps'.
This layer will get your heightmap (I used Filters -> Noise -> Perlin... and on my second try, remembered to set my colors to black and white, rather than black and dark green).
Add to this second layer a Live Lighting layer of type 'Directional', with azimuth and elevation set appropriately.
Set the pixel layer to 'overlay'.
This gives you a layer that can provide your bump map:
Hide the pixel layer to turn off the bumps.
Hide the lighting layer (and change the pixel layer's blend mode back to Normal) to see the height map.
Edit the pixel layer (height map) to change the bumps.
Add a mask layer to the pixel layer (height map) to turn the bumps on and off selectively.
(I tried cropping my sample file but the cropped version was bigger! So I went with the original)