Trouble with rice coal is it needs quite a pitch to run down the chute, about 40 degrees, that's not an issue for me most of the time since I have a hi-lift. The only time it becomes an issue is when I can't get close enough to the window. In ideal circumstances I can put it in a window 7 feet off the ground.
If the truck dumps and has a small opening in the tailgate you can utilize just set the chute up so the coal is going onto the chute. Hopefully it will have a enough pitch to go down the chute. Aluminum or stainless works best, any other metal such as steel will be impossible to work with unless it's corrosion free (i.e brand new) or have a very steep pitch. If the window is very narrow that will exacerbate the problem since you will need a chute with a very narrow end which slows the coal down even further.
If you don't have acess to something like that a slick hard plastic can be used. If the truck doesn't dump and/or you can't get enough pitch the only alternative is to make it so it has enogh pitch and shovel it onto the chute.
This is why I get paid to do what I do.