^ No. ENT's effects were not rendered at 480p. It was native HD. I think it was 720p. You may be thinking of the director's cut of TMP, whose new effects were
done at 480p (but can be re-rendered).
As for DS9 and Voyager, those shows did it just like TNG did: the effects were done on film, but transferred to videotape for editing, which automatically knocked the resolution down to 480i.