Hi Martin
A bit of imagination is needed.
One option is to have the coal truck raise, no coal fall out, then the full truck lower again.
Another option is to have coal in the truck actually emptying into the tender. However, gravity won't scale.
If in the prototype coaling tower the coal has to fall 80ft, then it will take the coal 2.2 seconds to fall.
( Remember s = ut + 1/2 a t2 )
In a 4mm scale model the coal will fall in 0.26 seconds which is a lot faster than the prototype, and it is unlikely that the protypical cloud of dust will properly materialise.
However, despite the above, as the falling coal is hidden for most of its journey, and if the truck itself is behind the coaling tower one might get away with it.
A coaling operation would certainly make an interesting application of DCC sound, with appropriate sound effects of machinery, coal falling, then crashing into the empty tender, etc and a reasonable sized speaker inside the tower.