Alright; I only get an empty window with the ok/save buttons, but before debugging all that:

Does yad even have a mechanism to report events on-the-fly?

I briefly skimmed the man page and as far as I can tell, it only reports user actions on exit.
If That's correct, you may need to go all the way to Gtk to get what you want.

If yad can write to file on-the-fly, run it in the background and read up on fork and similar)