Yeah, but I don't want a 16 colour greyscale view of what's on my Pandora screen at present, and will theoretically be on my Pyra screen. I support apps writing to a specific framebuffer that's designed for the target screen, even though that would be a little more work to get something working.
The drawback to that approach is compatibility would be programming/changes app by app within the device. The advantage is that apps that do address it directly would suck less. We're still not talking about speeds to impress or play games by, though. For the most part it is just enough information to be able to tell what your phone is doing and to tell it to do something else.
Just doing it by screen captures would be programming/changes OS by OS or at worst device by device. Yes, <=1 FPS based on screen captures scaled & converted to 16 shades of gray has it's limitations - but those limitations nearly match the limitations of the e-ink display.
How about this - do both.
Device connects to host via Bluetooth (security through code of 4 to 128 user chosen numerics input on both used as seed).
User needs to install an app on the host. The app would have two modes of operation. Default would be screen captures. However, have a handle for apps to engage with that allows the foreground application to directly address the on-host frame buffer for the handset device. This way the device is usable regardless of the application - but room is left for applications to be optimized to work with it.
Envisioning the hardware of this is the relatively easy part. The software... I wonder if this could even be done to an Android host without rooting. It would have to bypass the host's security screen after all and run with the host screen turned off. Getting it to work on the Pyra would conceivably be easier (less restrictive OS), but then we're talking about a niche of a niche population - not big enough sales to create molds for. It would need iOS, Android, Debian & Sailfish host apps, and the permissions requirements could be a bit rough.
[doublepost=1520014605,1520014363][/doublepost]Clearly we've gone down a rabbit hole and figured out we want something in the form-facctor of the, "Light Phone 2", but we want a different software stack - and no cell phone radios - and maybe an audiophile grade DAC and amplifier with a headphone jack ...
But the screen type & form factor for a handset/dialer - Light Phone 2 has that nailed nicely.