I don't know much of treating the bees - disclaimer.
But IF a TBH has dual entrances at both ends, why not use some forced air movement (apply the med at one end - slowly suck the air from the other end - that should pull the vapor/gas through the entire long hive).
Say, you notice that med started exiting from the entrance where you pull the air out - this means that the med has filled the entire hive volume - you now can stop pulling.
I don't know of the workable implementation of this or even feasibility.
But the idea seem logical to me - assuming dual-entrance TBH OR an asymmetric entrance (then you can create a hole at the opposite end and do the same pull-through process).
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