That is awesome :D I know it should work... it's too easy to make work today, but I can't help imagine being back in the 90s and showing someone classic MacOS running on a tablet. It would blow their mind.
A lot of e-ink devices just run Android so it's relatively easy to run whatever you want on them.
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.
I mean, classic Macs didn't exactly have turbo charged refresh anyways. Yes, things like scrolling chunks of text is probably faster on an actual first-gen Mac than an e-Ink screen, but they weren't really... turbo for graphics rendering.
https://imgur.com/a/kJRCR