Location: Hendersonville, NC in summers and The Villages, FL in winters
Is it possible to just use a large metric bolt with a 16 mm head and then jam two nuts together on the threads to provide a place to turn? Even better would be to make a quick weld on the nuts to the threads. This is what I use for the axle of my wife's VTX 1300.
Another option is a 16 mm coupling nut although this might be hard to find. If you did find one, they would likely be cheap. They are long enough that you could get one end into the axle hole and put a socket on the other end.
Others had the same difficulty finding the proper size, so they went with a coupler nut instead. You can see their post here.
I think that is the way I am going to go, but I hear they changed the newer years to a 14mm so it might be easier to find.
Why can't you just use a 5/8" or 16 mm socket to "grab" the portion of the coupling nut that is sticking out of the axle hole? The coupling nut is long enough that it could stick into a bottomless axle hole deep enough and still have plenty sticking out for the socket or wrench to "grab".