The e-ink displays aren't great for interactive applications, because the refresh rate is slow (ie, it takes about half a second to update the screen). A shell could possibly be made to work faster, with some modifications to the way it updates the screen, eg output that wraps around from bottom to top, instead of scrolling, so that the whole screen doesn't have to be rewritten.
The
Jinke company allows you to download the SDK and Linux source code for their e-readers, so you can do anything you want, but in the default install there is no easy way to access a shell.