Thanks for the safety heads-up!!
Please refresh my/our memory: what type of stove, type of coal, type of chimney etc??
In general, if a stove has draft, and the cover is on the hopper, there should not be any way that heat and oxygen can flow through the hopper to support a fire.
Now if the cover was left off, and the chimney draft was low, then I can see a fire backing up into a coal hopper.
My LL pioneer stove in my shop has VERY low draft, I basicly just vent it to the outdoors, the combustion fan pushes the air outside. Not a setup I'd use in a house with living, breathing creatures residing. This stove has moisture in the coal hopper, but I have run the hopper low, to the point of running out of coal many times, and never had even a hint of a fire backing up into the hopper. But, I always keep the cover on the hopper.
Let us know the details of your stove and setup.
Thanks again for letting us know about a potential safety issue.