[dmoreno]

Not exactly what you are looking for, but I created a web interface for ALSA sequencer, which effectively makes any raspberry pi what you are looking for.

For old MIDI DIN you would need a MIDI adapter or use the raspberry pi serial ports and some circuitry. Can be powered by an USB powerbank for example.

https://github.com/davidmoreno/aseqrc

I also added to the mix rtpmidi (https://github.com/davidmoreno/rtpmidid), and USB MIDI gadget (https://github.com/davidmoreno/midiconfigfs) on the PI, so then I can use the pi itself from one or several computers via Ethernet and USB.