For anyone who comes across this later: Norwegian, at least at JFK, does not allow lining up until something around 2 hours before departure (apparently, or at least something like that); the consequence was that to board the entire flight - esp. since it was international - all Norwegian staff were busy streamlining the process as much as possible, so not only was baggage size a non-concern, it felt like calling anyone out for oversized baggage would have slowed down the boarding process immensely, and was definitely not something anyone cared about paying attention to.

I'm relatively confident that I can fit my carry-on into one of those sizing cages - as long as the bars don't block the feet of it.