Idea for the structure of a simple Midi Input node.

midi_input_node.py

import bpy
from bpy.props import *
from ... base_types import AnimationNode
from ... data_structures import DoubleList

class MidiNoteData(bpy.types.PropertyGroup):
    noteName = StringProperty() # e.g. C4, B5, ...
    # This value can be keyframed.
    # It is possible but not easy to 'find' the fcurve of this property.
    # Therefor only the value in the current frame can be accessed efficiently.
    # In most use cases this should be enough, otherwise you'll have to find another alternative.
    value = FloatProperty()

class MidiInputNode(bpy.types.Node, AnimationNode):
    bl_idname = "an_MidiInputNode"
    bl_label = "MIDI Input Node"
    bl_width_default = 300

    # I'd suggest to bake one channel per node for now.
    # You can have multiple nodes of course.
    channelName = StringProperty() # e.g. Piano, ...
    notes = CollectionProperty(type = MidiNoteData)

    def create(self):
        self.newOutput("Text List", "Notes", "notes")
        self.newOutput("Float List", "Values", "values")

    def draw(self, layout):
        layout.prop(self, "channelName")
        self.invokeSelector(layout, "PATH", "bakeMidi", text = "Bake Midi")

    def execute(self):
        notes = [item.noteName for item in self.notes]
        values = [item.value for item in self.notes]
        return notes, DoubleList.fromValues(values)

    def bakeMidi(self, path):
        # Remove previously baked data
        self.removeFCurvesOfThisNode()
        self.notes.clear()

        # This function creates an abstraction for the somewhat complicated stuff
        # that is needed to insert the keyframes. It is needed because in Blender
        # custom node trees don't work well with fcurves yet.
        def createNote(name):
            dataPath = "nodes[\"{}\"].notes[{}].value".format(self.name, len(self.notes))
            item = self.notes.add()
            item.noteName = name

            def insertKeyframe(value, frame):
                item.value = value
                self.id_data.keyframe_insert(dataPath, frame = frame)

            return insertKeyframe

        # Add a first note with keyframes
        addKeyframe = createNote("First Note")
        addKeyframe(value = 2, frame = 0)
        addKeyframe(value = 42, frame = 100)

        # Add a second note with keyframes
        addKeyframe = createNote("Second Note")
        addKeyframe(value = 13, frame = 5)
        addKeyframe(value = 20, frame = 20)

    def removeFCurvesOfThisNode(self):
        try: action = self.id_data.animation_data.action
        except: return
        if action is None:
            return

        fCurvesToRemove = []
        pathPrefix = "nodes[\"{}\"]".format(self.name)
        for fCurve in action.fcurves:
            if fCurve.data_path.startswith(pathPrefix):
                fCurvesToRemove.append(fCurve)

        for fCurve in fCurvesToRemove:
            action.fcurves.remove(fCurve)