Why go to all the effort of cracking and parsing the XML file, assuming that you even can while he presentation's open in PPT, especially since you're already doing this from an add-in?
You mentioned that the user has selected a shape. Why not just tag the shape:
With ActiveWindow.Selection.ShapeRange(1)
.Tags.Add "Flippable", "Yes"
End With
You'd want a bit more code around that to make sure that there IS a selection, and if you want to allow tagging multiple shapes at one time, you'd need to put in inside a For Each/Next loop.
Then assuming a reference to the current shape to be tested in oSh
If oSh.Tags("Flippable")="Yes" Then
' Do your stuff
End If