Knuth and MacKay were the first to extent DVI, because it does not adequately support bidirectional typesetting.
Yes, and, as I already tried to communicate to you, this extended format (I assume you mean DVI-IVD) seems to me like a dead end, because at the time it was developed, over twenty years ago, there was almost no foundation for encoding bidirectional text; and it has never been updated to take Unicode's bidi algorithm into account.
Are you saying, Arthur, that Source to PDF using LuaTeX directly can proceed via Source to DVI using LuaTeX and then DVI to PDF using xdv2pdf (or some other tool) ?
Of course it can, depending on the information you put in your DVI. If you've been attentive to the explanations I sent earlier to this list about vertical typesetting, you'll know where it can fail. In this specific case, though, xdv's extensions would have been of no use at all: we would need some further extensions of the DVI format.
I've been told (by Hans Hagen) that even for pdfTeX this is not advisable.
It certainly isn't advisable. Nevertheless, in many cases, it's still possible.
I'm looking for a what might be called a Unicode savvy Device Independent binary format. And I'm looking for XeTeX and LuTeX to share code and ideas, when possible.
Hence, what you're aiming at is for LuaTeX and XeTeX to produce some common extended DVI format, not for LuaTeX to produce xdv, right? This is quite different since, as I said above, LuaTeX has some features that XeTeX's xdv can't account for.
I'm copying Jonathan Kew (XeTeX's developer in on this).
I know who Jonathan Kew is. Arthur