One last suggestion. I see the latex export backend takes any BEGIN_XXX and
outputs a \begin{XXX} environment. That would be almost what I was asking
for when XXX = equation or displaymath or whatever. Except that in that
case the inline preview wont work (for BEGIN_LATEX it was working just
because of the extra $$...$$ I added there). Again, this makes me believe
that in the same way a latex lang parameter activates native latex
fontification in *any* kind of block it should also activate inline latex
previewing. The relevant blocks here are SRC, EXPORT and any XXX exported
as a latex env.