Update for Monday, January 28th - I called Shimano and they are not going to be able to help me. The item is out of warranty and replacement rims are not available. Since my rear wheel seems to be OK, I may try to acquire a used matching front wheel at a reasonable price (say under $100 with shipping?).

But in the meantime, just out of curiosity I was going to try to apply a bead of metal epoxy (AKA “JB Weld”) over the crack just for the hell of it to see if I can get this thing to hold air tubeless.