Now that is interesting. I love learning something here all the time. So, they really need to dust all winter long also, even when they will be locked up in the coop for an extended period of time. Last winter was brutal, and the old farmers almenac says that this coming winter will be even colder.
Like cracrzy4chicks, I'm going to put a big kitty litter container in a corner of my hen house with wood ashes in this fall. I saw where someone mixed ashes with other things, like some sand and I can't remember what else. Actually I'll try it in the run first.
Like Reagan says, "trust but verify" I'd rather have the mess outside if it is one.
The girls don't make a mess. BUT, you'll need a deep container. I use a cheap one from Walmart, it's about 12"- 18" deep. That way they can't throw all of the ash out of the bin !! And they will throw it out of the bin, when they really get flopping & rolling around in it !!