I decided to try this with an itinerary we've already booked, and it does work, BUT that TA's site show very different availability than the HAL site. For example, the cabin I am interested in shows unavailable on the TA site, but available on the HAL site and the HAL site shows no aft verandahs available, whereas the TA site shows them ALL available. As someone else said, availability seems to be a State Secret!