I’m not sure that the renaming a PNG as a JPG to achieve the transparency effects is particular to the WDTV Live Hub, but if it’s a common “trick” for reflections and trans then why not add an option to save a PNG as a JPG right in TG. You could make a third option in the file I/O options that would save in PNG format but named as a JPG.
Such as:
JPG
PNG
PNG>JPG
Just a thought.