The way it escapes being an AOW is by not being able to be concealed, decidedly by the arbitrary 26"oal , If you make it less than 26" = AOW, over 26" and concealed no longer meets the definition of firearm. Not sure what guidance or more you are looking for seems pretty strait forward
The ATF is clear that it is a firearm under the GCA and not a firearm under NFA. They do say they get to make a new determination should you carry it concealed BUT it does not say it is an AOW if carried concealed. Do you have another reference after this guidance that does declare it an AOW under the NFA in the case of concealed carry? I could not find one.
you can read it hear.