This is difficult.. as I said in the comment on bug 37370, we find libraries don't want to visually show utf8 in the export menu, but do want Zotero to work. As such, completely denying the utf8 option, which is the default fallback when no format is passed too, breaks the Zotero browser extension.
Perhaps a possible solution is a second preference to allow Zotero specifically for export.. and use that to trigger an allow on utf8 export even if it's not selected in the UI facing preference?