Since this seems to be a common problem people experience with these stokers, is there any reason there isn't a small tube from the blower applying positive pressure to the auger tube a foot or so before the retort? This is standard on all bituminous stokers and it prevents just these problems.
My thoughts are that anthracite is generally, under good conditions, less likely to off-gas prior to arriving at the point of combustion in the retort. However, under less than ideal conditions (probably why there were holes drilled in many of these augers a few inches before they enter the retort) such as moist coal, poor coal, coal too large etc. there will be gasses migrating down the auger tube. Because bituminous coal is more likely to off-gas under most conditions (as well as generally being a larger size that doesn't "seal" as well) prior to entering the exact zone of intended combustion, all bituminous stokers head off this problem by a simple and very effective solution - using a small tube from the combustion blower to the auger tube about a foot or so prior to the stoker retort.
It seems as if this problem is both somewhat common and very unpleasant/potentially dangerous in its result, thus a complete and yet still very simple solution such as that used by bit stoker mfg'rs would be the ticket here to keep this from being a chronic customer complaint.