I suppose I should have just posted the entire thing :-) :But there were one
or two options. Here'sthe code with the oMMMD stuff inserted. I hope it's
correct:

Sub ModifyZipFieldName()

' Macro by PJ Jamieson, March 2004
' v1.1 Modified May 2004 to do a merge to a document
' This macro looks for a field name called "ZIP/Postal Code"
' in the header record of a mail merge data source and modifies
' the name so that Word automatically maps its ZIP/Postal code
' to the field.
' Written for use with Word 2003/Outlook 2003, but it may work
' in earlier versions
' Requires the "Microsoft Scripting Runtime" object library:
' You will need to use the VBA Editor menu command
' Tools|References to add a reference to this library

' This macro is intended to be used when a merge is initiated
' from Outlook. In that case, I believe the following assumptions
' can be made:
'   The merge data file is a Unicode format text file
'   (notice that by default it has a .doc extension)
'   Outlook reconnects to the data source and removes any
'   sort/filter options you may have defined

' The macro also assumes that it is OK to create/overwrite a
' file with the same name as the data source + ".tmp"

Dim oFileSystemObject As Scripting.FileSystemObject
Dim oSourceStream As Scripting.TextStream
Dim oDestStream As Scripting.TextStream
Dim sHeaderRecord As String
Dim sMergeDataSourceName As String
Dim vMergeType As WdMailMergeMainDocType
' v1.1 don't need this line any more
' Dim vMergeDestination As WdMailMergeDestination
Dim oMMMD As Word.Document
On Error GoTo finish

Set oMMMD = ActiveDocument
With oMMMD.MailMerge
 If .MainDocumentType = wdNotAMergeDocument Then
   MsgBox "This document is not a mail merge main document"
 Else
   ' Save merge type
   vMergeType = .MainDocumentType
   ' v1.1 don't need this line any more
   ' vMergeDestination = .Destination
   sMergeDataSourceName = .DataSource.Name
   .MainDocumentType = wdNotAMergeDocument

   Set oFileSystemObject = CreateObject("Scripting.FileSystemObject")
   With oFileSystemObject
     ' This assumes the file is Unicode format (I think)
     Set oSourceStream = .OpenTextFile( _
                           FileName:=sMergeDataSourceName, _
                           IOMode:=ForReading, _
                           Create:=False, _
                           Format:=TristateTrue)

     ' Verify that the required string exists before going any further
     sHeaderRecord = oSourceStream.ReadLine
     If InStr(1, sHeaderRecord, "ZIP/Postal Code") = 0 Then
       MsgBox "Could not find the field name 'ZIP/Postal Code'" & _
              "in the merge data source." & _
              vbCrLf & "You will need to match fields manually"
       oSourceStream.Close
       Set oSourceStream = Nothing
     Else
       ' create the output file
       Set oDestStream = .CreateTextFile( _
                         FileName:=sMergeDataSourceName + ".tmp", _
                         overwrite:=True, _
                         unicode:=True)

       ' Make the substitution. The field names "ZIP" and "Postcode"
       ' seem to be recognised automatically by Word

       oDestStream.WriteLine _
         Text:=Replace(sHeaderRecord, "ZIP/Postal Code", "ZIP")

       ' copy the rest of the file
       Do Until oSourceStream.AtEndOfStream
         oDestStream.WriteLine Text:=oSourceStream.ReadLine
       Loop

       ' Close everything and replace the old file by the new one
       oDestStream.Close
       Set oDestStream = Nothing
       oSourceStream.Close
       Set oSourceStream = Nothing
       oFileSystemObject.DeleteFile _
         filespec:=sMergeDataSourceName, _
         force:=True
       oFileSystemObject.MoveFile _
         Source:=sMergeDataSourceName + ".tmp", _
         Destination:=sMergeDataSourceName
     End If
   End With
   Set oFileSystemObject = Nothing
 End If

 ' Set up the mail merge data source etc. again
 ' You may find that you need to save and restore
 ' other settings
 .OpenDataSource Name:=sMergeDataSourceName
 .MainDocumentType = vMergeType

 ' v1.1 don't restore the original destination,
 ' just make it a newdocument
 ' .Destination = vMergeDestination
 .Destination = wdSendToNewDocument
 With .DataSource
   .FirstRecord = wdDefaultFirstRecord
   .LastRecord = wdDefaultLastRecord
 End With
 .Execute Pause:=False

End With
oMMMD.Close SaveChanges:=False
Set oMMMD = Nothing
Exit Sub

Exit Sub
finish:
 Close #1
 MsgBox "Error " & Err.Number & _
        " when trying to modify the ZIP field name: " & _
        vbCrLf & Err.Description
 Err.Clear
 On Error Resume Next
 Set oSourceStream = Nothing
 Set oDestStream = Nothing
 Set oFileSystemObject = Nothing
 Set oMMMD = Nothing
End Sub

<snip