Gearhound,
Yes, that's my point, that there is no standard, so we created one, along with agency-wide training, within our community of instructors and students, and it integrates with the standard back gas config.
On the long hose question, the Z-System is set up exactly like back gas doubles, so the long hose is always in your mouth and that's the donatable regulator. Then you (the donor) go to the necklace. Both second stages are both always on. The clipping and unclipping in Basic-6 is a drill to be able to stow the long hose in the event it's not needed (right side failure). The only time we would ever donate something that's clipped is in the MX Rebreathrer config (but please everyone, that's not for this thread).
As side mount moves away from solo diving and more into the hands of team divers, the trap, as I see it, is a zero-viz out-of-gas scenario. If your muscle memory is from backgas doubles, you find your teammate, tap tap tap on the reg that's in his mouth, and take it. He goes to his necklace, and you sort out your problems. I don't know how to resolve the situation where you go tap tap tap and the donor is breathing a necklace.
In any case, I found the first 20 posts in this thread really interesting, since everyone is creating their own system, unlike DIR, which is based in consistency. With back gas doubles, we could dive with pretty much anyone trained by one of the DIR agencies or instructors and everything was pretty consistent.
I look forward to seeing where it all goes in the next few years.
Cheers,
Jeff