Outlook doesn't provide any direct way to reply to a message with a custom form. Is a little VBA macro a possibility? In a 25-user environment, it might be easier to do that than build and deploy a complete COM add-in. The code is relatively simple:

Sub DoMyReply()
   Dim objOL as Outlook.Application
   Dim objItem as Object
   Dim objReply as Outlook.MailItem

   Set objOL = CreateObject("Outlook.Application")
   Set objItem = objOL.ActiveExplorer.Selection(1)
   If not objItem Is Nothing Then
       If objItem.Class = olMail Then
           Set objReply = objItem.Reply
           With objReply
               .Bcc = "yourBCC@somehwere.com")
               .Categories = "your category"
               .Display
           End With
       End If
   End If

   Set objOL = Nothing
   Set objItem = Nothing
   Set objReply = Nothing
End Sub

See http://www.slipstick.com/dev/vb.htm if you need VBA basics.