Extracting DVD-Video surround audio to DVD-A??

First you need to find a ripper that can defeat the copy protection and get the DTS or DD track off of the DVD.

Having done that, you need to decode the DD/DTS to the basic WAV format that discWelder supports. I've used foobar2000 to do that, but if I recall correctly I could only persuade it to output a single 6-channel WAV file, whereas discWelder needs six 1-channel WAV files. But any audio editor that supports multi-channel WAV should be able to make that last conversion.

All a bit of a pain. Maybe someone can suggest an easier route.

It's very easy.

1. Download DVD-Audio Extractor http://www.castudio.org/dvdaudioextractor/ free for 30 days. Use this to extract the DVD-V audio track into a 6 channel wav file.
2. Download DVD-Audio Solo http://www.cirlinca.com/products.htm free for 30 days. Use this software to write the 6 chanel (5.1) wav files to a blank DVD-R in DVD-A format.

It's that simple!!!!!!! Works great in my TL. Love Elton John's live DVD-V's from Best Buy on this.

Anyway to do the reverse?

None that I know of. No one has a software program that breaks the DVD-A encryption, and allows for ripping of the tracks. Besides, you don't need to, as all DVD-A's already have the downmixes on the DVD-V already.

The first part will (usually) only extract the Dobly Dirgital stream (11/12 comnpression) and not the DTS stream.
Best bet is to find a copy of DVDDecrypter somewhere and go into Stream Processing mode, extract the DTS file, and then look up a lovely little tool called "Tranzcode" and the "Tranzcode GUI".
This will take the DTS stream & demux into 6 mono 32 bit Floating Point WAV files.
Run these through your DAW and get fixed point 24/96 files.
Best of all is that DVDDecrypter will also generate an IFO file with all the chapter points in HH:MM:SS:FF or HH:MM:SS:CCC (The latter is for discWelder use) so you know where to place the markers.

Then use Cirlinca's DVD-Audio Solo.

Be aware there is a hidden "Gotcha" in DVD-Audio Solo though.
It will allow you to create a disc using 5.1 at 24/96 without MLP compression. This is out of spec, and whilst it will play on some players, because of the bitrate being a whopping 13.8Mb/sec, it is far from guaranteed to play.
You may want to consider running a Sample Rate Conversion (If the DTS stream was at 96KHz) to 48KHz.
Odds are quite high the DTS stream will be at 24/48 payload though.

Actually, the DVD-Audio Extractor does extract the DTS stream! Just click on the DDS heading, and a drop down list will show the DTS stream to choose. this will then change all the tracks below the heading to the DTS stream choice.