This actually isn't difficult. At least on my 6416, the S-Video output jack is active even in HD channels, in fact I have a JVC S-VHS VCR connected that way.
Just run an S-VHS cable from the back of the 6416 to the S-Video input on the DVR. S-video isn't 480i, and it isn't component, but S-Video is capable of about 400 line resolution. By contrast, SD over the air is limited to about 330 lines. So while S-Video isn't quite as good as 480i, it is very close, and a lot better than composite video.
At any rate, I can watch HD channels as SD full screen by routing the s-video signal through the VCR and into the S-Video input on my TV. I can also record them as SD on the S-VHS VCR, so there is no reason to think you cannot do the same thing with a DVD recorder.
So you should be able to record HD TV to your DVD recorder via the S-Video output on the 6416 while watching HD TV.