Com um pouco de pesquisa sobre os fóruns, encontrei o código abaixo que funciona bem perto do que eu estava procurando:
Sub CountWords()
Dim MyRange As Range
Dim CellCount As Long
Dim TotalWords As Long
Dim NumWords As Integer
Dim Raw As String
'ACTIVE SELECTION
Set MyRange = ActiveSheet.Range(ActiveWindow.Selection.Address)
TotalWords = 0
For CellCount = 1 To MyRange.Cells.Count
If Not MyRange.Cells(CellCount).HasFormula Then
Raw = MyRange.Cells(CellCount).Value
Raw = Trim(Raw)
If Len(Raw) > 0 Then
NumWords = 1
Else
NumWords = 0
End If
While InStr(Raw, " ") > 0
Raw = Mid(Raw, InStr(Raw, " "))
Raw = Trim(Raw)
NumWords = NumWords + 1
Wend
TotalWords = TotalWords + NumWords
End If
Next CellCount
MsgBox "There are " & TotalWords & " words in the selection."
End Sub