Although I have not done this myself, I will one day. I have noticed that on my Rene Herse bike wheels the spokes are tied with a copper wire about the size of old solid telephone line. My guess it is around 24-26 ga and then soldered which when fluxed properly tins the entire wire giving a nice appearance. I think good ol 60-40 tin/lead would work fine. Should be pretty simple to get from a home depot or some other hardware outlet.
I also noticed that the tying was a wrap around both spokes at the cross by first laying a small length of the wire on top and parallel to the rear of the inner most spoke, about 3/16 of an inch. Then slowly wrap the wire around both spokes at the cross and perpendicular and over the piece of wire you laid parallel. This holds the wire in place while you wrap it 4 or 5 turns then cutting it leaving again 3/16ths over and tucking that in the front using some needle nose. Use a good flux and low heat and solder away. Clean excess flux with water as it can be corrosive.
Ray Homiski
Elizabeth, NJ