Not as cool but if you really want to run System 7.5 on an e-ink display you can already run a Mac emulator on an ebook reader today.
Also the e-ink display running classic is very reminiscent of one of the early LCD displays in this old thing: https://en.wikipedia.org/wiki/Macintosh_Portable
Refresh rate depends on how much of the screen is updating at once. Scrolling a whole page is pretty bad, updating a small region is reasonably quick. Some devices have an 'A2 mode' that makes it somewhat faster by just doing black/white instead of grayscale.