You could use something like
http://www.videohelp.com/tools/BDtoAVCHD
It makes a type of dvd called an avchd, which most bluray players play.
And actually, you can make a real DVD-Video (vob's) with HD content, and it will play on HD-DVD and Bluray players, I've tested this. You are limited to the crappy mpeg2 codec though. Playing it on an SD player might crash it.
I've played HD divx on a divx player, and it showed me the upper corner of the HD video.