I have the BL2840 as well and loving my very comfortable home and garage. As temps have dropped close to -20, I have filled it 90% of the way. The only side effect I have seen is that the coal bed prevented air from coming thru from the bottom and started to choke the fire causing temps to drop until I raked it open a bit. Once I did that temps popped right back up. I am burning mostly green beech, so your results may vary with type of wood and how well seasoned.
Just sharing my experience and learning its behavior.
I think I have read the suggestion of not filling wood past the firebrick so that the acidic ash doesn't contact the metal directly. Personally, think that's overthinking the potential problem. I say fill as needed to keep it burning hot for up to 12 hours. Keeping it hot prevent condensation or any moisture from staying in the firebox.
Again, by no means an expert on anything but that's my logic to the question. Time may prove me wrong.