It will all get worked out eventually, but I personally think it will be quite some time before its working correctly, reliably and completely enough that the "port" concept can be counted on LPX.
Scripter will have to be modified, for example, so that in a typical Scripter script, you can use a "port" atttribute from Event objects, etc.. WIth multiport, you have even more channels funneling through a single Scripter script...as many as 768 total channels could all be funneling through a single Script, and you will need to be able to identify Event objects by both their channel (which it already does) and their port, in order to decide what to do.
That's just one example. But what about the enviornment, transformers, etc.. There is a lot of old legacy code that frankly could end up broken.
What about the articulation set framework? It will need to become port aware also.
Most of the other midi plugins, if they ever did anything related to channel, they will need to become port aware.
Piano roll, event list, and on and on...many places in LPX that will need to be updated to support and know about a new port attrtibute of event objects.
I am not that optimistic that Apple is going to fully enable everything we need enabled for multiport to work, at least not right away. They will get there eventually, hopefully, but its going to take some time and probably several broken iterations of LPX. IMHO. In my view this is much more difficult thing to add to LPX then people realize because of all the old legacy code. Take the enviornment, it is still litered with decades old bugs that have never been fixed and probably will never be fixed, a lot of it was coded back in a time when code had to be made extremely efficient in order to deal with older and slower machines... Code like that takes advantage of every bit, and is often difficult to change. To this day, the enviornment is completely unaware of ArticulationID, for example. Transformer still can't see it or set it or do anything related to it. It gets piggy backed onto Event objects somewhere along the line and passed through, but that's about it. Will that be the case with the port attribute? I would say highly likely yes.
Anyway we shall see how it goes, but I just personally think we all need to be a bit patient with AU3 and multiport, LPX has a lot of changes that need to happen. And by the way, LPX can only support 16 channels of audio coming back from the AU3 plugin also...so...there is that limitation as well.