You can probably do it with a keyboard paired to a server/RPi that emits the keystrokes to the Roku ECP API, if having that second device is acceptable.

This might be possible now. I think the better option is having a hardware device that acts a bridge between a bluetooth keyboard and the Roku.

On my rokus, I am able to use my phone as a remote via the roku app. This includes typing on mobile via my phone's keyboard. Makes logging into things much easier.