Here's another similar idea. I do have access to a lathe, but I'm trying to save a little bit of time, and turning oak can be a real dog.
Regarding the attached picture, in my mind, the left and right would be recessed 1/4" in on all four sides (the cross-hatched area). I can't think of a way to do this safely or accurately. A router might be able to take off the material, but that seems a bit like a long shot without having horrible results.
I might try to find some pearlescent diamond inlays instead of using walnut. I'll come up with something and post the results.
P.S., thats for the links, I wish I had those before I resawed a 16/4 8' oak beam! It would have saved me much of a headache.