Yes! I have wanted something like this for a while. It would be perfect for blogs, or any situation where you need to make a page but then sometimes add some custom HTML too. Embedding Markdown inside of Haml just seems like the wrong direction; the strategy outlined here makes much more sense.
I’ve been doing this for years using nanoc with kramdown support, which I’m quite happy with, so I’m not 100% sure how much of this idea overlaps with it.
Fulrthermore, kramdown also allows you to render your content for different output formats beyond HTML, which I use for creating both a PDF and a web page for certain content.
I never used Haml, but I use Slim (which looks similar) and have lots of frustration with it.
Adding Markdown to this feels like nightmare. I don’t know, maybe Haml is better designed than Slim. I have similar experience with yaml, it’s also indentation-based and has lots of syntax sugar. Had the same problems and frustration with it.
Editing erb templates (with help of editor extensions) feels so relieving after Slim.
I would never mix Slim and Markdown. It would be simply impossible to differentiate one from another. Haml and Markdown have not that many clashes in syntax.