I understand unwrapping would be ideal but I am not very profficient with unwrapping and the collapsed object corresponds to a 300K polygon object, where there is one unique material, one unique id, and geometry with terraces, external walls, window openings etc (like if had isolated a 4 storey 40m long building to see just the brickwalls and balconies). There are all sorts of things happenning in the facade and even unwrapping to later paint would be quite time consuming...
I am now thinking of duplicating this object, subdividing it to my desired paint resolution, and using UV mode on it to cover it fully with my FP. I can then use a paint tool in max to select all parts of meshes I want covered with my veg and delete the rest of this mesh- which will be occluded from rendering.
I cannot think of anything more efficient..