Now I'm amenable to the view that "original" art is only ever a mixture of what's already been done, but are there alternative perspectives on originality? Is originality a prerequisite to AGI? Are there any proposed frameworks for measuring whether a system is interpolating points within its training set, or, what is the opposite of interpolation?
You've just described Art. All art is in response to previous art, usually a variation of the same, but with a new twist (that tries to stand defiant/transcendent of the past).
There is a very long debate in the sciences about what intelligence and creativity is. The questions will probably only be answered conclusively - if at all - when we know exactly how the brain and consciousness work in detail; this will still take some time.
Nevertheless, scientists who specialize in the subject already have well-founded expert opinions. In terms of creativity, there are established tests, and apparently ChatGPT performs very well here; see e.g. https://neurosciencenews.com/ai-creativity-23585/.
I do not find the point that models merely "interpolate between points included in its training set" (in many dimensions, mind you) troubling, since essentially human creativity, as we understand it today, is also a combination of what we have learned and experienced. The conclusion that "mixing what has already been done" is "unable to create anything original" is wrong and contradicts experience. Just take a look at the patent database to see how many inventions there are that merely arrange or combine something existing in a slightly different way. A somewhat prominent theory of how inventions are accomplished is e.g. https://en.wikipedia.org/wiki/TRIZ.
You don't need to know Japanese, just click into the images. But warn you that stuff being drawn inside is something 1400-years-old elf that never get old.
https://itest.5ch.net/fate/test/read.cgi/liveuranus/16660895...