Private Declare Function SendMessage Lib "User32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Long) As Long
Const EM_LINEFROMCHAR = &HC9
Const EM_LINEINDEX = &HBB

Function RTFPosicion() As String
Dim LineaActual As Long
Dim ColumnaActual As Long

LineaActual = SendMessage(rtf.hwnd, EM_LINEFROMCHAR, -1, 0&) + 1
ColumnaActual = rtf.SelStart - SendMessage(rtf.hwnd, EM_LINEINDEX, -1, 0&) + 1

RTFPosicion = LineaActual & "/" & ColumnaActual
End Function

Y en los eventos MouseUp y KeyUp pones algo como lblPosicion = RTFPosicion ()