Kudos and thanks to all who contributed to building this handy tool. I used this to perform a complicated booking that otherwise would have taken a long time by phone, assuming I could get an agent inclined to help.

Incidentally, in other threads about Matrix, some folks complain about Matrix showing incorrect availability for QR. I saw that problem in spades. One interesting twist: When ITA was returning a response proposing a higher booking class than I wanted (based the availability I could see in ExpertFlyer), I found I could right-click on this userscript's Priceline link, copy to a text editor, edit the BookingClass parameters to give the booking classes I wanted, and paste into the browser's Address Bar. This actually saved me about 15% -- the difference between I and D on one fare component. Perhaps others can use this trick. I can imagine JS to make this possible in the UI, but probably more trouble than it's worth, especially because QR thankfully seems to be one of the few carriers with frequent discrepancies in ITA.