We recently added support for embedding LaTeX expressions (and really, any dynamically-generated DOM) within Markdown template literals to Observable notebooks. So you can say md`I like ${tex`\\KaTeX`}`, and KaTeX renders its logo, which is then embedded within the generated Markdown. I’m admittedly biased, but I love the ease of writing Markdown combined with being able to inline arbitrary DOM for math, sparklines e…
I know observable as a whole is not open-source so I understand this may not align with the best interests of the company. Also observable is fairly new so I'm sure you have a lot higher priority features on the release radar. I honestly wouldn't mind paying for an application even if it was an electron app.
Anyways good luck with Observable! I think its a great notebook format with a great API.