A few years back, we had an Oak log with a small hollow in it, about like the one end of that log of yours did there...but shorter, it was maybe 5-6' long. We stuffed a plastic peanut butter jar full of Tannerite in it and lit 'er off...KA-BOOM!...instant "fence posts" Actually, it split the log in to pretty even quarters...which ended up about 200' apart!
The hollow logs I hate are the ones where you are felling, get into the tree a bit, and then suddenly gallons of nasty rot-water comes spraying out all over you. Nothing like getting a nasty-bath of rot-water mixed with sticky sawdust. Man I hate that.