Just FYI, Orbitz/CheapTickets allow per-flight booking class specification. I'm not sure how easy it is to search like that, but if you can find the flight on ITA, it's easy to book using some URL construction (see tool in sig).
An example I got to pull up (not sure if this is what you were going for, but I did force it to Y/J so its $$):
Orbitz Example