Classic ASP is just plain old VBScript. CreateObject creates a COM object using the classid/progid: CreateObject("ADODB.Connection") or CreateObject("Scripting.FileSystemObject").
Classic ASP can use COM objects that are actually .NET objects... but only if they have been built specifically supporting COM interop. Most of the internal .NET stuff was not built supporting COM interop.
See: http://msdn.microsoft.com/en-us/library/zsfww439.aspx
If you just need a stream object (not necesarily a .NET System.IO.Stream object) then I'd recommend ADODB.Stream.
Also not that in ASP/VBScript all variables are variants. This makes things like an array of bytes tricky. You can have an array of variants no big deal, and all the variants could be bytes... but you can't create an array that can only hold bytes. To make matters stranger... if a COM object returns a SAFEARRAY of bytes then ASP/VBScript is happy to use it.