That sort of setup exists and is quite common - I use a 4x20 lcd, rotary encoder and sd card setup with marlin firmware, and my reprap is not tethered to my pc. Admittedly I do have to connect it to flash new firmware, but I hardly ever do this, and I need a pc anyway to generate the gcode and move it to the sd card. The self-flashing firmware would still be a very neat addition to the marlin firmware if you wanted to share your code though.
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