You can take a chisel and a hammer and chisel out an opening....time consuming, but it will get the results you want eventually. You can also take a large drill bit and keep drilling holes in the log, right next to each other, until you have drilled out the whole area. You can use a chisel too and chip out between the holes. Mark the drill bit with a piece of masking tape so you know where to stop drilling each time.

If hubby has a band saw, he can cut the log in half, cut the center out and then glue it back together. We do this for birdhouses, but it may not stay together if you are planting in it.

The tip of a chainsaw will work too--just be careful.
I have a small electric one that I could use.

My dh placed charcoal and some lighter fluid on stumps.When it burns down a bit,did out the center.you won't need it to deep to plant sedum.Lol