"The Complete Aquarium" by Peter W. Scott has a chapter on creating an Amazon River Acid Backwater Pool biotope. It recommends mixing coal dust with medium-dark colored pea gravel. Aside from washing the coal dust, it doesn't note anything else about its use. For fish in this biotope, it recommends several kinds of Apistogramma, neons, cardinals, and some other luminescent tetras. Also Corydoras catfish. For decor, bogwood, and for plants, swords, Cabomba, and Myriophyllum.
I'd say it would be safe to use, in moderation. Being from Western PA, I of course can tell you that coal mining results in acid mine drainage. But if the intent is to create an Amazon Acid Backwater, then it would seem to be alright in moderation.
Personally, I really like the look of medium dark gravel mixed with black substrate. But I used Eco-Complete when I did it.
If you go this route, I'd say be prudent. Start off with some hardy, cheap fish and be sure that nothing bad happens to them.