Good morning Jerry.
Although I haven't had the privilege of bantering with you before, I'm certainly glad to hear of your recovery.
To your question of sealing the end grain.
In your case, may I guess that the oak has probably been cut for awhile and is basically dry. Perhaps surprisingly, the end grain in your case is not likely to move appreciably differently than the centre of the boards.
What I always used end sealer for was when cutting lumber on my sawmill, and this was latex paint, the purpose for which was to slow down the drying of the end of the boards. Once dry, the boards would not have much seasonal movement. In this case if the ends are not sealed, splitting of the ends of the boards can be expected.
You mention glueing cedar across your oak panel. Do I gather that the cedar would cover 100% of the oak panel?
Two things come to mind here, the first being what kind of glue will you use? And secondly, I wonder if the end to end movement of cedar doesn't come very close to the expansion/contraction of oak across the grain.
If they are very close, then the panel will move very little. If the oak has more movement than the cedar, then the panel could bow very slightly on a seasonal basis. I doubt that would be much.
Essentially what you have there is a two ply version of plywood. So if one wood moved more than the other, some bending may happen. If you were to use thinner oak and then add two layers of cedar, you would now have a more stable panel I believe.
As for the depth of the rabbet, I would try something in the order of 5/16" to 3/8". You would want to avoid having it too tight for obvious reasons. A little loose would be OK.
I didn't check on the expansion rate of oak, but the cedars move quite appreciably. I think a lot of it is going to depend on the initial m/c of the oak.
OK, fess up...which one of you clowns stole my sig? It was right here a second ago.