As a follow up on this work @agoose77 last posted:
@agoose77 and some of the ExecutableBooks team have been working on jupyterlab-myst which brings these ideas to life with additional MyST Markdown. This supports the same inline execution as jupyterlab-imarkdown (in the same metadata as well) and also incorporates it into other MyST Markdown styles like admonitions, tabs, proofs, etc. Works with widgets, text, math, and inline matplotlib images as sparklines.
In this implementation we went with the {eval}`x.sum()` syntax, which is already present in MyST Markdown.
A blog post about our implementation is here: