This sounds like it could be a great inspiration for AI-based projects.
I understand that for their flagship production, they want people to draw true 4k all digital. But for most projects, I'd wager that a good AI upscaling and cleanup can take a hand-drawn paper character to a usable 4K plate.
Similarly, I don't think the contractors doing in-between frames need to work in HDR. It should be good enough to upconvert their images from SDR to HDR based on the adjacent HDR keyframes.
As for the background images, I myself already felt frustrated a few times when trying to get my 12MP SDR vacation photos displaying nicely on a 4K HDR TV, so solving that problem might even have a consumer angle.
There is actually precedent for that. Waifu2x [1] is an image upscaler for anime-style art based on deep convolutional neural networks. As far as I know, it's mostly used by people who want to upscale fan art they found on the internet. It works pretty well, though, and it (or similar) could serve as a basis for the kind of workflow you're talking about.