If I remember correctly, table cell overset is not stored in the same way as "normal" overset. You have to iterate the table model to find it.
My point is that InDesign may not actuallty calculate overset on cells it before something triggers it to do this - like clicking on the page. Have you tries to iterate the table right after the document is opened ?
Thanks for thinking about this. I'll give that a go next time I'm looking at this plugin.