Changing the pixel size of Xvid AVI, but nothing else

So, my DVD Ripper of choice is Xilisoft, I find it the fastest option, with enough options, but nice enough GUI to be extremely usable. My problem with it, as I found out to my displeasure is when ripping 16:9 DVDs to Xvid avis. Now, they give you an option of the aspect ratio, but that bears no impact on the avi you end up with, no matter what you choose, it gives you a 4:3 avi, with 4:3 pixel size.

Now, I found a great program, MPEG4Modifier, which changes the header information of the files, to say 16:9, instead of 4:3, so that programs like Windows Media Player and the like play it properly, which is great, but that wasn't what I wanted to use the rips for. I've an Archos MP4 player and wanted to watch the rips while travelling. Problem is that the Archos doesn't take any notice of the headers, obviously, it just plays the file how it is, so when it's on there, it still tries to play as 4:3 and all messed up.

So, after trying all the other way rounds, and not really interested in going through each and every DVD I ripped and doing it all over again, I was wondering if there's a program that'll hopefully batch convert loads of files, but not re-encode, just change pixel size. Is this even possible, I don't know? I've used AviDemux a little bit and that's great for just copying the stream you put in, but if I change a setting, I think it needs to fully re-encode, which I think would be a massive waste of effort.

Just to give you an example of this, as AviDemux also sees through my hack, here's an example of what it sees, as you can see, I've put a fake filter on the file, just to get a preview. So even though its size is 720*576, it's coming up as a stretched, 4:3 image:

Thanks for any help you'd be able to give and I'll take on board any and all reasonable suggestions.