Dude, you do realize that asbestos is illegal to sell, right? It causes serious lung diseases.
I had to take asbestos tiles out of a basement apartment in a house left to me from my grand dad so I know this for a fact. I couldn't rent the place till all the asbestos was taken out. And you best beleive that I was wearing a good particle mask when I took that stuff out, solid form or not I wasn't about to take chances.
Wood, or bark, isn't my first choice because it absorbs and holds water. So while your dirt may be dry, the tree bark in the dirt could still be full of water. You can use it if you want, but don't say I didn't warn you if you start running into over & under watering problems.