I thought this might be a relevant place to mention that, after about a three-year hiatus, DVDAuthor has resumed active development. This is the common open-source authoring engine used by every single one of these open-source GUI frontends. It already did most of what you wanted to do, albeit modulo various irritating bugs and limitations and the like.
With Scott Smiths blessing, I have taken over as principal developer. I have already done loads of bug fixes, incorporation of long-standing useful patch submissions, and other enhancements. I see it as the goal of DVDAuthor to support every publicly-known feature of the DVD-Video spec.