A few of them are under development and probably will stay that way until the actual bike is out and the parts that are available doesn't really need the bike to be physically there for testing unless the g310r is so far out of the norm for BMW.
I don't think aftermarket shops will be getting the bikes ahead of time unless they know someone in BMW who can help them out. They generally have to wait like everyone else to get their hands on one. Maybe the first one to hit the showroom floors but not before.
Doubt it will happen here but looking at other bikes turn around time hasn't been that bad, so give the average turn around time, the waiting process shouldn't be too bad. Although some guys might find a need to fab up things like a fender eliminator before official parts come.
You'll probably have to ship a few parts from wherever the g310r is most popular. Areas with a high saturation of low cc bikes tends to have more aftermarket companies who are willing to make parts for them. In NA, not so much IMO.