I was trying to apply this on irrlicht terrain some weeks ago, what I got was this:
Im able to set an alpha map to a texture but when rendering it only shows the last texture I've set to draw, u can try to draw sand texture the last one and it will b drawn but not grass texture.
I've used example 12.TerrainRendering with VC8 to try it so if u wanna try it just replace main.cpp from there and extract media content to media folder.
main.cpp
media.rar
This is my first irrlicht attempt, Help would b apreciated, thanks!