Somebody asked a similar question last month:
We thought this was not possible, so I would be interested to see the plugin that does this. Is there a demo version, do you have a link?
I just downloaded the trial. I think they just replace the text every time it changes. The reasons I think are because If you try to change the text:
1. The document is shown as modified (* after name in title bar)
2. An undo is generated.
3. You can sometimes see the text change very briefly in the transparency preview.
Thanks that was reassuring - I was thinking there is some kind of magic code that is not evident in SDK APIs. Is there a call back on the textframe when text changes?