How about a display that had a large e-ink screen plus a small LCD of maybe 2-4 lines at the bottom?
Go old school for editing. Use something like TECO or ed or Rob Pike, David Tilbrook, Hugh Redelmeier and Tom Duff's Unix version of QED [1].
The small LCD is for seeing the command you are currently typing and a little command history.
With those editors you entered editing commands but did not see the results until you asked for them. You'd tell the editor to show you the current line plus say 10 lines before and after. Then you'd give it commands to edit the current line, such as telling it to change the text "float" to "double", or telling it to insert a new line before the current line, and so on. When you had done enough changes that you needed to refresh your notion of the current state of the file, you'd ask it to show you again.
Even the older generation, slower e-ink screens would be fast enough for that kind of work.
Maybe instead of putting the small LCD display at the bottom of the e-ink screen, make it a separate unit that can attach to the e-ink screen or attach to the top of your keyboard or stand alone somewhere if you prefer.
Also programmers tend to type pretty fast after getting some experience. It's disconcerting enough when code completion slows down the editor, not sure i could stand a screen that can only refresh every full line.
