Comment by simedw

If the attribute describes the current note, I would first try adding it as another field/head on the note event. For example:

  NOTE(
    pitch,
    delta_onset,
    duration,
    attack_velocity,
    release_velocity,
    ...
  )

More global things might be better modelled as a control event:

  NOTE(...)
  | CONTROL_SUSTAIN(delta_onset, value)
  | CONTROL_TEMPO(delta_onset, bpm)
  | CONTROL_PROGRAM(delta_onset, instrument)

The harder part might actually be finding enough good training data with all of those attributes represented consistently.