At the moment, I'm looking specifically at what we need to worry about
at a low level. For example, the current expl3 code does not take any
notice of direction, which is probably right for something like \hbox:n
(follow whatever is going on around it), but should be documented and
deliberate, not just something we've ignored. So what's important at
this stage is much more the concepts than trying to write any code,
although any thoughts on what is required for RTL support at the 'base
level' are of course welcome.
For the boxes in luatex you can change directions: \hbox dir TRT{...}
What you say fits in with what I'd already suspected: for RTL work,
we've be better only supporting one set of primitives, the Omega ones.
Yes, because if you want to typeset TeX--XeT, you will find that you have to patch a lot due to the limitaions/bugs of the engine which is wrong.
For pdfTeX that's not an issue: I doubt very many people use pdfTeX for
RTL.
Well, there are two groups of people. The first group use ArabTeX which does not make any use of TeX--XeT and it works with Knuth TeX too. The second group also are Hebrew and Arab users; some of them still use babel.
XeTeX is a bit more 'interesting': I guess the existence of bidi
means that people are using XeTeX for 'real life' RTL work, despite
limitations.
Considering bidi has improved the situations and made things cleaner and simpler, yes.
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