For the keyboard I use a CH340+CH9329 cable, the former is a USB to UART converter and the latter is a UART to USB-HID converter. Feels unnecessarily complicated but hey at least it's only $3. It's effectively a USB "keyboard" controlled by another host.

Then I use a ~30 line Python script to forward key press to it. I really hope someone make a better integrated solution (or me having time to do it), though.