I am not sure I understand your question. Or perhaps I am missing some possibilities in Terragen.
I assume it can render a normall pass from a  spherical  or looking down ortho camera.     But for a game I need a tangent space normal map matching the exact low poly shell model  with low poly tricks like  polygonal fins with alpha on the top of a  mountain ridge for example to   mimic complex silhuette   with all the trees. buildings and small details. 
 Projecting all this from a shperical camera is  working only for very distant flat background    and for someething 3d and closer to main part of a game scene (middleground)  I need   more specific to exact low poly geometry projecting angles.   
ps. Perhaps I could coupe with world space normal colors  to convert them into tangent ones later in soft like Substance Designer   but still I need a specific angles of projection and the "cage" to set it   for evry target polygon vertex/corner.   
I just think that instead of transferring all the complex hi res geometry and materials from Terragen to MAx or MAya  it woud be so much easier to just import low poly shell model retopoed over micromesh  and  use baking ray tracing  to produce necessary textures.