ZED agrements seem to be a little strange at Air Japan. Typically, a ZED ticket is valid on any ZED carrier but here, you are limited to only 4 carriers per ticket. CO, as well as AA and NW are Zed carriers, but for some reason we do not have an agreement with them. According to the manual, the only carrier that Air Japan has interline agreements with going to the US is United(and of course ANA).
Over the last 6 years at my previous job, I have done well over 100 segments using ZED / ID 90 tickets and I never got stuck. Sometimes it took some very creative ticketing (meaning around the world in the opposite direction), but you can always make it work if you plan ahead and are flexable. Of course, this causes an increase in transit time and less time at home.
While I am no fan of United, they are the best way to get to Japan from most of the US. You could very easily get from Houston to Chicago and probably fairly cheaply if you planned ahead and bought a confirmed ticket. You then have 2 UA flights a day from Chicago plus an NH (ANA) flight that you could use ZED tickets on. Other options are getting to SFO, LAX, or SEA also having 1 to 2 flights daily to NRT on UA and NH.
That being said, if you cannot do a multi-leg commute, you will have to buy a ticket or look elsewhere. Houston would be easierto commute from if you were able to do a 2 leg commute but I totally understand your reasoning.
** take my opinion with a grain of salt. I am still here in training and dont have any actual experience using ZEDs here at AJX. It shouldnt be much different than any other carrier though.