Hmm, I don't know. I'm trying to think of the logic path in the code.
I think that the alternate encoding (Unicode, for example) is used if the filename or comment on any of the entries, necessitates it. I don't believe using regular ascii filenames and comments on all the entries, with a unicode comment on the zipfile
as a whole, will result in a unicode-encoded zipfile. I think.
I don't have the code in front of me so cannot review it just now to verify this theory.
If I am right, then the way to enforce Unicode is to use a unicode character in the comment on a zipfile entry (ZipEntry.Comment), as well as on the zipfile itself (ZipFile.Comment).
I'm not saying this is desirable behavior. I'm just guessing that this edge case isn't well handled, and I'm suggesting a possible workaround.