As Duff-Man said, DVD-Audiophile is the way to go for DVD-A creation. It works best when you use FLAC files (doesn't like the header on certain WAV files), so best to losslessly compress the audio to FLAC first.
SACD authoring is not a real option. With DSD being a proprietary format, there's no real consumer or even prosumer way to listen to DSD recordings (by, say, a Tascam DSD recording deck or the new Korg recorders coming out) without converting to PCM first. SACD authoring is only, as far as I know, done by sending the DSD RAW files to an SACD mastering house. That's a very expensive way to do things.
Personally, I don't care about listening to 24bit audio in the car. There's too high of a noise floor to make the better detail worthwhile. While I love my 24bit recordings at home, I can even live with a high bitrate mp3 (gasp!) in the car!
EDIT: just to clarify - I'm talking about recordings made by us in DSD format, NOT a commercial SACD. Of course we know how to listen to a commercial SACD, but there's no real way to listen to a DSD recording made by us in the field/studio/whatever without converting to PCM first.