Asbestos is really only hazardous when it's airborne. I think the safest thing you could do is to cover it to a sufficient depth that it won't be accidentally exposed in the future by you or the chickens, and them avoid unnecessary digging (i.e. no root crops) that might stir it up. The fibres are too large to be taken up by plants.
Elementally, asbestos is silica, magnesuim, and oxygen, all common elements in the soil, and it won't form any chemically harmful breakdown products.
If the disks were treated with other chemicals, that might pose other problems.