I understand what people are getting at, as it's not really that big of a logical leap. I think the fact that it is somewhat of a stretch, but still "in the lines," is exactly why it is a "trick": it isn't doing anything particularly invalid or hacky, it's just not necessarily what you'd imagine when reading the RFC. Content negotiation to me is more about serving the optimal content to a given agent, not really about selecting modalities for different use cases based on different types of user agents.

I think both cases are "valid" although I think it is inherently less tricky if the document talking about and previewing the dataset is referenced via a separate URL from the dataset itself. (Which, of course, entirely mitigates problems like Apache Spark having HTML in the Accept header.)