One way is to set the camera at the same position as the light source, and point the camera at the entity you want a shadow on.
Set the ambientlight to zero, and hide all lights and other entities (except the one you want a shadow on, then set the camera cls colour to white.
Then do a renderworld, and copy this to a texture. Unhide everything and set the ambient lighting back to normal, and the camera cls colour back to what it was, use the texture you just rendered for the shadow.
You need to use a certain texturefilter on it tho, so the white disappears, and the black remains as the shadow. Then simply renderworld again.
You might not want to do this every frame though, because it's slow copying frame VideoRam like this.
Set the ambientlight to zero, and hide all lights and other entities (except the one you want a shadow on, then set the camera cls colour to white.
Then do a renderworld, and copy this to a texture. Unhide everything and set the ambient lighting back to normal, and the camera cls colour back to what it was, use the texture you just rendered for the shadow.
You need to use a certain texturefilter on it tho, so the white disappears, and the black remains as the shadow. Then simply renderworld again.
You might not want to do this every frame though, because it's slow copying frame VideoRam like this.