Hi Alex,
I'm building Starboard Notebook (some demo noteboks on the frontpage here), a notebook system for the browser. Much like dataflow it also features notebooks which are defined as plaintext files that are easy-to-git (click view source on the top right on the Starboard homepage).
Different than this project however is that notebooks are entirely static: you don't need any server running at all. Your system would embed the notebook in an iframe (and talk to it to for instance inject any secrets).
Starboard is built around dynamic importing of functionality: you can arbitrarily add new "cell types". For instance it supports Python in the browser, and recently I managed to hack together talking to a remote Jupyter kernel.
It would be really cool to support a cell type that is "Observable", I would love to work together on this. What it will provide Dataflow is a runtime that is sandboxed by default: a Starboard notebook runs entirely on a different origin in an iframe (editor included!), as well as a live notebook environment with code editor (as opposed to editing the source text files) - so pretty similar to Observable!
Now imagine interop with Python in the browser.. A workflow could be: "Input CSV file" -> "run some Pandas code in Python through Pyodide" -> "Visualize using Observable cell"... Super powerful stuff!
I can ramble for a long time about why this would be great, but I'd love to hear your thoughts on perhaps adopting Starboard as the notebook "runtime" here. Happy to answer any questions as well of course! (happy to chat somewhere if its easier)