One problem solved, an other one appears...
I created a simple textured 3D model and exported it as a NIF, then tried using it as "decoration". Worked fine... Then using the "city" XML file, I defined it as a small building (pointing to the same NIF file). The building is used by SMR when laying out cities, but all that is displayed is its 3D shape with everything in plain black. The textures are gone.
Anyone knows if SMR is stricter on how textures are used in city objects as opposed to decoration ones? And if yes, what other constraints do I have to respect? This is weird