custom.py

from bge import types, render, logic

class CustomTechnique(types.KX_RenderTechnique):
  def __init__(self):
    super(CustomTechnique, self).__init__()
    
    self.normals_shader = types.BL_Shader(...)
    
  def apply(self):
    # render all the meshes with a specific shader
    # If the shader override (param 2) is null, use the material
    # We can have a function to render a single mesh and bind the textures to a shader
    # self.renderSingleGeometry(index, shader)
    # and self.getGeometryCount(scene)
    # so, for i in range(self.getGeometryCount(scene))
    # To bind textures:
    #    self.bindTextures(mesh, shader)
    self.renderGeometries(logic.getCurrentScene(), self.normals_shader)
    self.postProcess()

def main(cont):
  render.renderTechnique = CustomTechnique() # if renderTechnique is null, render normally, else, use the technique