Moving forward, after the first sanding pass, I installed the J-Channel on the lift-gate and the pop-out panel. That also allowed me to add the final sprinkle of screws that were waiting until I slid the J-Channel under the Homasote. Next will come another round of fill for the new screw holes and all the ones that didn't get fully filled. I can also cut the pop-out panel out of the 1/2" birch and have it ready to paint with the rest of the table, the size is fixed. 
This should keep the Homasote from getting beat up over the years on the edges. All the edges should be covered when we're done.
