Being a LONG time TeX/LaTeX user (since 1987) and have writing & published >100 papers, 2 books, and a PhD Thesis, mostly all written in Latex, I don't think you're going to get an Evernote/LaTeX mix that's going to be perfect and make everyone happy. I've experimented a lot with the various offerings and I usually simply return to using stand-alone LaTeX and sharing the resulting PDF.
MathJaX is very nice, is used on a number of Blogs, is used by a number of Physics/Math journals for HTML versions of papers.
Some ideas for the developers to chew upon.
adopt a "cell"