Well, I don't know how this would work. I havn't dealt with MIDI yet, but as far as I am aware, one has to create a virtual MIDI port for each plugin that wants to send MIDI. I might be wrong though.
Still, the latency can neither be estimated by the plugins involved nor by the host. It may be cool for a live situation, but when bouncing it can be totally off. Latzter would cause too much trouble to even think about it.
Funnily there have been proposals for MIDI connections in AUs for some 7 years or so. A dedicated bus that keeps the host aware of connections would circumvent all problems. Just like it does in VST-land (where I think you still have the problem that MIDI is only passed on within the same channel strip, but at least it's working...)
Seriously, I'm not trying to avoid the topic. All I'm saying is that the status quo is far from ideal, and to my best knowledge, not worth bothering. And I gave up asking for improvements in the area, maybe in 2005 or so.
Maybe some motivated user should start a petition or something.
😉 Urs