I just stumbled across this thread. I don't know that I really have much definitive to offer, but I do think that scottw's and bloke's (last) comments are pretty much on target. I just spent a long time trying to convince one of my sons not to use a butcher block countertop as a cutting board and that there was no finish that would really work for that. Oh well.
I can understand and empathize with your desire to preserve this thing. So the question is how to do it.
First, you need to stabilize this piece of wood. Almost impossible, given what you've got, but ... If I were to do it I would first cut a "base" for it that would be attractive. This in itself is difficult because the damned thing is so big. But there are a couple of possibilities. One is (if you've got a decent planer) to create a large enough base out of several boards edge-glued (or tongue and grooved) together. The other possibility is to use a good grade of 3/4" plywood (say birch or oak) and then put an edge on it so it looks decent. (If you don't have a lot of tooling to do this, you could use the "iron-on" edging you can get in places like Lowes. It works fine if you're not trying for really high-end products.)
Then glue the stump slice to the plywood with epoxy.
Next, you have to do something about that crack. Instead of trying to cover it up or make it disappear, emphasize it! Same for any other more minor cracks that have appeared in the interim. Take advantage of it as an "artistic feature". Fill it with lab metal (
http://www.jdindustrialsupply.com/labme48ozcan.html" target="_blank" target="_blank" target="_blank) which will turn it into a silver "accent". Sand down the surface. Then seal the whole thing (top of the stump slice, edge of the stump slice, exposed portions of the plywood base, whatever) with some sort of sealer. Probably water-based poly would work as well as anything and be easy.
So ... the result could look pretty good. But is it "food safe"? No. Not strictly speaking. You couldn't use it as a cutting board (which I presume you don't want to do anyway). I wouldn't put food directly on it. But you sure could put a plate on it and it would probably look pretty good.
Just some thoughts.
Old couch where the dog sleeps...should have cropped that out!
