Really nice scene. I like the lighting. However for some reason I feel like we need something ont he right side. A bluff. Or a little cliff, or something.
PS if you use Hannes trick and stick an inverted plane into the sky you can actually use the enviro light modulator for PT.
Here is my solution. Luminosity is very low for very little initial effect, but ramping up enviro light on surfaces, etc, will respond. You could tint the luminous colour if you wanted too.
<terragen_clip>
<plane
name = "Enviro Light Plane Fix"
gui_use_node_pos = "1"
gui_node_pos = "0 -120 0"
gui_group = ""
enable = "1"
visible_to_camera = "0"
visibility = "0"
visible_to_other_rays = "1"
cast_shadows = "0"
center = "0 0 0"
length_a = "10000"
length_b = "10000"
edge_vector_a = "-1 0 0"
edge_vector_b = "0 0 1"
surface_shader = "Luminosity"
displacement_tolerance = "1"
render_method = "0"
sorting_bias = "0"
>
<default_shader
name = "Luminosity"
gui_use_node_pos = "1"
gui_node_pos = "200 -60 0"
gui_group = ""
enable = "1"
input_node = ""
gui_use_preview_patch_size = "0"
gui_preview_patch_size = "1000 1000"
diffuse_colour = "0 0 0"
colour_image = ""
colour_function = ""
translucency = "0 0 0"
translucency_image = ""
translucency_function = ""
luminosity = "0.009999999776 0.009999999776 0.009999999776"
luminosity_image = ""
luminosity_function = ""
specular_roughness = "0.8"
specular_roughness_image = ""
invert_specular_roughness_image = "0"
convert_specular_roughness_image_to_linear = "0"
specular_roughness_function = ""
specular_roughness_model = "3"
metalness = "0"
metalness_image = ""
convert_metalness_image_to_linear = "0"
metalness_function = ""
reflectivity = "0.75 0.75 0.75"
reflectivity_image = ""
reflectivity_function = ""
reflection_tint = "1 1 1"
index_of_refraction = "1.5"
displacement_direction = "1"
displacement_multiplier = "0.01"
displacement_image = ""
convert_displacement_image_to_linear = "0"
displacement_function = ""
displacement_offset = "0"
opacity = "1 1 1"
opacity_image = ""
use_alpha_channel = "0"
invert_opacity_image = "0"
opacity_function = ""
alpha_from_colour = "0"
alpha_key = "0 0 0"
key_tolerance = "0.1"
image_projection = "4"
projection_camera = ""
unpremultiply_colour = "0"
unpremultiply_metalness = "0"
unpremultiply_translucency = "0"
unpremultiply_luminosity = "0"
unpremultiply_reflectivity = "0"
unpremultiply_specular_roughness = "0"
>
</default_shader>
</plane>
</terragen_clip>