For a previous job, I wrote a text extractor for PowerPoint files. From what I remember about the format, even your text changer would not be an easy job. The file is basically a bunch of serialized objects stored in an OLE stream. The text extracter walked through the object tree and recognized the text objects.

It might be possible to modify the strings in place because they are simplest objects. But it would be a pain to insert the changes in the file. I forgot if there are offsets and sizes that would need to be adjusted but I suspect there are.

I would worry about the reliability of the binary editor. I bet it would be brittle to changes.