Hm.. I just realized; If you want to be able to change midi information in one MIDI-clip and have these changes made for all instruments at once, this is actually something that can be done..
You can copy the midi clip and then use
paste special onto the other tracks. In the
paste special-window, select the option "linked clip".
By doing this all the MIDI-clips will be linked and all the changes you make in one MIDI-clip will be applied to the other tracks/clips as well.