Edit: You work on that stuff, right? Then this armchair experting feels silly, just imagine it's for other readers.

It seems much more practical (if a little less power-efficient) to implement the no diff -> no refresh logic for screen regions in the display hardware. The RAM and logic for a display-side framebuffer can't be expensive today, a couple of Euros for the extra board space and chip(s). If that stuff takes off, just additional transistors in the all-in-one ASIC.

For the whole screen, that more or less already exists in laptop hardware: "panel self-refresh". HDMI and DiplayPort might need a new extension or something? Is there anything?

The Embedded DisplayPort standard has had the panel self-refresh feature since 2011, and the partial update feature since ~2015. I found a press release from Parade in early 2017 for a TCON supporting the partial refresh feature. I don't think there's anything missing from the ecosystem other than a strong impetus to use those features.

Yes! I work with the Modos team. You’re exactly right - ideally we want region-based refresh hinting. The SDK supports some region based features - we’d like to extend that functionality.