No, this is the right place. :-)
Is there any planned support for expl3 syntax?No, not really as it seems nobody from us is using it.
I would imagine AUCTeX development is geared more toward authors than developers (and rightfully so)., so this isn’t too surprising :) You should give it a try though – it’s pretty neat. It’s an interesting language.
Could you please test this expl3.el style file and check if it DTRT?
It definitely changes the syntax table (as reported by C-h s), but the syntax table doesn’t have an effect on word-based movement (though it does work with symbol-based movement, which makes more sense. Wouldn’t be too difficult for the end user to bind M-[fb] to their symbol equivalents when in any sort of `expl3` mode. So yes, as far as pure navigation, this does the right thing. :)
What would be the right way to go about adding the syntax highlighting? If there’s any existing framework in AUCTeX to do this (i.e. particular to csnames), I could try to write something up that does what’s needed.
P.S. The ‘rx’ macro makes some of these regular expressions much less of a headache: `(rx "\\" (+ (or (syntax symbol) (syntax word))))` :-)