What Wolke here is after, is something that would act like the "low power mode" screen that N9 has. You know, the really dim display mode that consumes very little power and can display your messages, battery status, clock... whatever.

The way I understand this is, it is not about the keyboard other half at all, it's about what you put underneath it, on the underside.
Basically it would be a display of really low power, on the bottom of the keyboard other half, so when the keyboard is closed and the phone is laying on the table face down, you can see all the notifications on the display.

I'd like to have something like that too, but instead of LCD I would like it to be an E-Ink display. Granted, that'd need a backlight to be seen in the darkness but on normal lighting conditions the contrast and current consumption would be unbeatable.

So, here my answers to the questions above:
1.) Very feasible, no problem at all making somethong like this.
2.) There's no need to do anything so complicated like that, i2c bus is addressable so you can control several devices with it concurrently.
3.) Both bandwidth and power requirements are OK, no problem with that. There are several microcontrollers you could use, if you want to use programmable devices. There are also several single-solution dedicated chips for that. There's no problem with thickness, all of these can be made as thin as you like it