I've never used chaga, but there are a few things you can do.
1. Email Randy and ask.
2. Figure out how to make a chaga tea, cool it down, then add some to a beer to figure out some dosing guidelines.
Once you've got that worked out, I think it is fine to either add it to the boil or make a tea and add it to the fermenter. You could also wait for the beer to ferment out, then pull samples and figure out how much to use to dose the keg. Then dose the keg.
Bottom line, there's a lot of different ways to do it and none is obviously more right than another. Absent anyone with actual experience using it, you'll just have to try it and see how you like it, then make adjustments for future batches.