Okay, so I think there is a patch that might do what you want although it's not perfect.
It needs this patch available here : viewtopic.php?t=83374
It allows trains to be shunted (detach/attach wagons and cars).
You would need to set the wagons you would want to be empty in a shunting yard, use a full load order with, like, 6 of your wagons, then get back at the shunting yard, take 6 more wagons, full load again, then take 6 wagons again, etc.
It's a bit complicated, but once you've got it it's very easy to understand.
It's not based on time, but rather quantity, but it's still great.
Also, if you like the idea of shunting, you can use OpenGFX+ Trains (available in Online Content Download, search for it) which will allow refitting of trains at stations, and thus allowing your train to go carry wood, then steel, then goods, then steel, etc. It will allow more realistic things.