Funny that I have a similar situation for May 15 IAH-NRT, with seat map showing only two seats assigned but J9/C9/D0/Z0 availability. The C fare fluctuates between C0 and C8 or C9, but always J9, D0 and Z0 on May 15, which falls on a Sunday. From past experience, Z seems hard to come by on weekends (Thu - Mon), but has a better chance of being available during mid-week (Tue - Wed/Thu). Therefore, I'm confirmed with a connection through SFO in Z all the way on UA, who has plenty of J9/C9/D9/Z9.

I was just about to call CO and see how the chance would be with Z on the non-stop CO7 IAH-NRT. From reading the above, it seems like it's of no help to call.