I did think about this, but I like the idea of being able to tighten up the joint if necessary in the future. The timbers were harvested around 4 years ago and have had two winters of central heating, but at 20cm thick (8"), I'm not convinced that they've stopped moving altogether. The post is holding up a handrail, one end of which is firmly locked into a beam, but I'd like to avoid any wobble.
I do like the idea though. It would make things visually neater. I'll let it sit in my head for a day or two and see if I can come around to it.