I will echo Davids response and can only comment from personal experience sawing from our own woodlot and having processed several "cookies" for peopel (including one recently in the image).
In my experience unless you have a species with deep lobes in the bark (redwood, often times some cedar) that allow for a lot of contraction there is simply no way your going to be left with anything other than a cookie full of checks, cracks, and splits along the rings, that may or may not remain stable.
You have to think of your cookie like an outer ring that is going to shrink very little as compared to the core of the cookie. As the interior heart wood shrinks there is no other outcome than for it to develop splits and cracks anywhere and everywhere.
I dont even think a shop built tank filled with PEG could save a large round. It would seem to me your option is to let it do what its going to do and deal with it at that point.
As far as what MC, thats no different than any other wood product. If there is water in the piece, it will push anything other than a breathable oil finish off the face. So your looking at sub 10% for a film finish.
Not sure if It will work but this was a pin oak crotch recently brought to us to surface on the CNC that was a nostalgic tree. Didnt seem worth fooling with and had been dried slow and inside for years. I was shocked it didnt fall apart.