If you want to simply tag a single document with every 100th word then I would just use Highlighting to make it obvious where these are. A macro method to add the highlights should probably firstly remove any existing highlights but that might not be what you want.
Code:
Sub TagEvery1000thWord()
Dim lWord As Long
ActiveDocument.Range.HighlightColorIndex = wdNoHighlight
For lWord = 1000 To ActiveDocument.Words.Count Step 1000
ActiveDocument.Words(lWord).HighlightColorIndex = wdPink
Next lWord
End Sub