A similar build is arturo182's Handheld Feather Wing, which uses a Blackberry Q10 keyboard [2] Though it doesn't have a processor of its own it can be plugged into Feather compatible boards that do (like @groguard's Giant Board, which runs Linux [3]). Makes quite a nice package!

I ended up going the other way and building a (split) 60% keyboard with a screen and a Pi in the middle [1]. It's long enough that you can sling it over your shoulder on a guitar strap, so it somewhat fails the pocketability test.

[1] https://github.com/borgel/commute-deck [2] https://hackaday.io/project/165450-keyboard-featherwing [3] https://www.crowdsupply.com/groboards/giant-board