@foone regarding Raspberry Pi working as keyboard and input device, there's USB4VC: a board that let you connect modern USB (and bluetooth) input devices (keyboard, mouse and gamepad) and then the Raspberry Pi emulates old protocols for vintage computers: e.g. PS/2, AT or XT keyboard, PS/2 or Serial mouse, and Gameport joystick.
(With optional remaping, i.e.: USB gamepad input events are mapped to key presses. Though that specific combination fails the 26 buttons criteria you mentionned)