Looking at random flights in April that is what I'm seeing so you are correct. The random date I looked (April 25th - "the perfect date", if you get the Miss Congeniality reference ) shows JFK-HNL as being operated with a 76L (767-300 with 36 D1 seats, no PS seats). Granted anything on the schedule that far out is really more of a placeholder, but it is correct that you could buy Main Cabin and upgrade to Delta One with an RUC (if it's showing as available for upgrade with RUC) since there is no PS cabin and by securing it now, you should remain in D1 even if there's a schedule change to an aircraft with PS as long as your RUC has been properly applied and ticketed.

ETA: Looking at other routings, it appears DL doesn't yet know what it's going to do for PS on the HNL routes because even routes that are being operated with aircraft that have a PS cabin, such as ATL/DTW-HNL both being operated by the A330-300 that day, are not showing PS as being an option for sale and instead the PS cabin is currently what's being sold as "C+" despite true PS being an option to HNL this year so I wonder if/when DL will load PS fares and sell the PS cabin as PS on these routes.
​​​
I would recommend NOT booking the two tickets together to start. I would recommend booking the ticket you plan to upgrade first and you should be able to apply the RUC to confirm into Delta One as you work through the booking process. Then book the second ticket in Main Cabin. It's the same net effect of booking both in Main and then calling to split the PNRs and then upgrading one of the tickets and all that happens here is more hassle than if you just booked ticket 1, applied the RUC online during booking, and then booked ticket 2. Clearing ATL/DTW/MSP/JFK-HNL to D1 with an RUC (especially if you can do it from Main Cabin) is about as good as it gets in terms of RUC use.