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.