OpenInkPot can be configured as an X11 device, see
Connect Via X Server. However, this would have to be via USB, because OpenInkPot does not support devices with Bluetooth or WiFi. Since almost all EInk readers are actually running Linux (like OpenInkPot), this capability is likely possible on any of them. On all, it requires access to the system files (i.e. would need jail breaking first on some) and enough Linux knowledge to setup an X11 server.
Programing could be as simple as generating an 600x800 image file and sending it to the device using the xloadimage application. X11 is not commonly used on Windows PCs, but it is possible to do so (e.g. using Cygwin/X). Note that since you can also run an X11 server on your PC, it is possible to create and debug your app without even buying an EInk device. The server can open in a 600x800 window on your PC screen, showing what will later on be displayed on the Eink device.
For EInk devices with WiFi and a web browser, it might instead be possible to do this by setting up a web server on your PC and pointing the EInk device's web browser to the server. This might not involve any programming at all on the EInk device. Once again you can test the concept using any web browser on a PC (although the browsers on EInk devices tend to be very basic).