There is no built in way of doing grass, outside of the usual static meshes and mesh groups. You can go off into the weeds with shell/fur rendering, volume rendering, etc… but that will all take a bit of custom development and probably a few hours spent reading theses and siggraph papers. I did a brief experiment with a scheme that uses a static mesh of billboards constantly centered on the player (thread: Grass/vegetation mapping repo: https://github.com/JTippetts/Urho3DGrassTest ) with mixed results. With a bit of artistic finesse it could maybe be made to work.
As far as querying the weight map, that is a simple matter of loading the map into an image and sampling a pixel directly. The map will have one color component mapped to your grass base texture, so you can just test that component vs some threshold and enable/disable grass at that spot depending on the result.