I'm not sure its authentic, to tell the truth.
If it is authentic, it would be very difficult to find who Mr. Honma was/is. The name is fairly common, and there is nothing identifying the school or institution where he taught.
There are no taboos that I know of regarding the handling of this kind of flag. The souls of soldiers aren't considered to "reside" in the flag.
You might consider sending it to the Obon society to see if they are interested in trying to locate the former owner's relatives.
This assumes the flag is genuine, obviously.
I understand the Japanese Government also has opened an office that seeks to repatriate these sorts of items from overseas.