According to http://www.webvivant.com/zipping-epub-files.html
the mimetype file should be stored in the ePub (just a ZIP with some XML and XHTML inside) with no "extras". Seems most readers do not care, but validators report things like "Extra field length for first filename must be 0, but was 36 !"
I checked even the code in ZipEntry.cs and ZipEntry.Write.cs and there doesn't seem to be a way to request that. Do you think (admittedly rather specific) option could be added.