Background: I work in a field where the use of LaTeX is common but still far from universal. So far I have been lucky in that all my coauthors in the past have been fellow LaTeX users, so collaboration boiled down to creating a shared Dropbox folder with the .tex and .bib files we were working on. Now I'm starting a collaboration with two colleagues who use Word, so they have proposed to collaborate with Google Docs. There are various reasons why this is a bad idea, the main one being that writing this paper is going to require doing things that are easy in LaTeX but difficult and time-consuming in Google Docs (or in a standard word processor, for that matter) ---e.g., Greek letters for variables, assorted math/logic symbols, trees (in the graph-theoretic sense), or frequent crossreferencing in the running text of numbered examples.
Conflict: One of my colleagues has already said he has no interest in learning LaTeX. On the other hand, I don't want to go hunting through the Google Docs character map every time I need to insert a non-Latin character.
Question: Is there any collaborative writing software that allows including LaTeX tags and environments in a Word-like document? For example, when I'm writing semi-informal things like lecture notes, I can get by with markdown and then generate a pdf with pandoc. I don't know of any online services with similar functionality.