What about setting the sun_light (which is directional) but using a special material for the model, which makes it non-sensitive for the sun_lights direction?

For a quick test:

A model usually gets the default material: mat_model

You can try the following instruction (also in-game):

vec_set (mat_model.diffuse_blue, vector(0,0,0));

This should remove the bright spots, while leaving the ambient intact.
If it works you could simply define a new material with the diffuse component set to zero.