Certainly this is feasible in realtime. You'll have to write the wavetable synthesis logic, but most of the rest of the plumbing is available in NAudio already. I would suggest implementing WaveProvider32 and having 8 instances of a custom ISampleProvider inside it to render each "track" or "channel" of audio. You'll also need to mix the channels inside the WaveProvider32.
You should probably seriously consider whether you need a "MIDI-like" implementation or just a simple mixing audio file player (which could support WAV, MP1 / MP2 / MP3, or Vorbis with minimal code on your part).
You should probably seriously consider whether you need a "MIDI-like" implementation or just a simple mixing audio file player (which could support WAV, MP1 / MP2 / MP3, or Vorbis with minimal code on your part).