Does not currently exist.
Pi Zero or Arduino Nano, using Samplerbox or some other soundfont player (must be sampler ie audio files) for a piano or other instrument because synthesized sound files have not been able to compete with the sounds of actual instruments. However, someone could emulate the sound of a piano key striking (at various strengths) as a osc formula, it looks like).
This should be small, powered by a AA (with it's own charging module), so you can tape it onto the top of a midi piano. Optionally, it should have a low-power LCD / eInk display, for selecting sounds. And an audio out jack.
As a second tier capability, it should record, either audio or the midi played. Further tier, having playback. Further tier, having ability to play along with a playback while recording new performance in a second, third, etc, track. Fourth tier, external audio recording.