I didn't realize they use copper melt wire. Copper melts at a much higher temperature so the copper wire fuse will have to get much hotter (approx 2000 deg F for copper vs 500 deg F for normal fuses). This would be a hurdle for the fuse designer but apparently they have gotten over it. I worry about the large resistance curve that the current must climb to get up to the melt temp. Resistance goes up with temp so as the copper heats up the resistance goes up. I'll not speculate on how this is addressed but it must be or the fuses wouldn't work.
So I agree there will be grain in the wire, but I'm not convinced that it is directional when it comes to conductivity. If it were, then wound transformers, with perhaps miles of wire, would show directional performance when leads are reversed, and they don't. Long transmission lines would perform differently in opposite directions. Now AC reverses 60 cycles so current flow is more complicated, but there are also 500KVDC transmission lines. So I can't prove a negative but lets just say I'm skeptical about the directional electrical characteristics of wire.
I still have my concerns that the melt wire has to be so small that it presents a resistance (otherwise it wouldn't heat up and melt) which limits current. To me, the overcurrent relay (such as the SDFB) is the way to go.
Jerry