Ha! I mentioned in reply to @stefanv that I think the DAP might be overkill / unnecessary indirection here, with just invoking the kernel directly in mind. However, as you point out, user_expressions would seemingly be even better. How prescient of you and the IPython team!
Or do you see a need for a custom cell type in the document format itself?
I think we do need a new cell type, because Markdown cells don’t have outputs field — we can’t store any mimebundles for a Markdown cell.
Long term, I think a proposal a la Notebook Cell-Type Generalisation would be best. Maybe if I work on this at some point, I’ll do both at the same time!