Bingo. I got it to work.

It was in the burning. Even though the opening wizard for K3b didn't include DVD video project, I was able to find it in some sub-menus of the New Project menu. There's a lot in there that's not listed in the opening dialogs.

This is why I hate wizards.

So, for future reference:

In K3b, don't use the wizard. Go to "File" > "New Project" and select "New DVD Video Project."

It will lay out the DVD project for you with the VIDEO_TS and AUDIO_TS folders. You just drop the corresponding contents into them. You must already have the vob's and stuff created by another program.

More than likely, what I was missing was some filesystem header or something.

Thanks a mil'!