Gave some further thought to this, and here is another approach you may want to consider. If you're not interested in a catalog of previous IDs, then you could simply use a custom document property to store the last ID that was used.
In Word 97-2003, you can add a custom property by going to "File / Properties", choosing the custom tab and assigning a name and value there. Adding a custom document property in Word 2007 is a bit more buried and off the top of my head, I think it's "Office Button / Prepare / Document Properties", choose the little drop down box for advanced properties and you'll get the same ol' pre-2007 dialog.
In the example below, I called mine simply "DocumentID" and assigned it an initial value of zero.
The relevant bit of code to update a Custom document property is:
ThisDocument.CustomDocumentProperties("DocumentID").Value = NewValue
As a proof of concept, I created a .dot file and used the following code in the Document_New() event:
Sub UpdateTemplate()
Dim Template As Word.Document
Dim NewDoc As Word.Document
Dim DocumentID As DocumentProperty
Dim LastID As Integer
Dim NewID As Integer
'Get a reference to the newly created document
Set NewDoc = ActiveDocument
'Open the template file
Set Template = Application.Documents.Open("C:\Doc1.dot")
'Get the custom document property
Set DocumentID = Template.CustomDocumentProperties("DocumentID")
'Get the current ID
LastID = DocumentID.Value
'Use any method you need for determining a new value
NewID = LastID + 1
'Update and close the template
Application.DisplayAlerts = wdAlertsNone
DocumentID.Value = NewID
Template.Saved = False
Template.Save
Template.Close
'Remove references to the template
NewDoc.AttachedTemplate = NormalTemplate
'Add your ID to the document somewhere
NewDoc.Range.InsertAfter ("The documentID for this document is " & NewID)
NewDoc.CustomDocumentProperties("DocumentID").Value = NewID
End Sub
Good luck!