One way to use an ARM board inside a mechanical keyboard without a screen would be to use text-to--speech output through an earpiece, e.g. with the Speakup scre

One way to use an ARM board inside a mechanical keyboard without a screen would be to use text-to--speech output through an earpiece, e.g. with the Speakup screen reader for Linux. But probably the only people who would want to use that are blind hackers. For that setup, a Raspberry Pi would probably be better than a BeagleBone Black, since the Pi has built-in analog audio output (though it's kind of crappy). I guess some of the Allwinner-based boards would work too.