Mr. Moderator, if this is better in UA, please feel free to move 
I need to transport my cat in-cabin on United HNL-BBB-CCC. I have already checked and there is "pet space" on the flights in both Y and F.
But I will be beginning this itin with a from-a-neighbor island flight (separate ticket -- the cost is literally $800 less than booking that segment as a UA codeshare. It's a peculiar market) on HA.
Many times I've had HA inter-line my luggage at my departure airport to a UA itin that begins at HNL.
I really don't want to have to exit and re-clear security when I connect in HNL to my UA flight.
Do I pay my in-cabin pet fee to UA over the phone? Will they send me a receipt to show the FAs (will the FAs even ask?) Or is this a, ahem, peculiarity where one could have the "pet reservation" note in the PNR but since not actually checking in with UA wouldn't get dinged?
Or is all of this moot because I WILL have to leave security to check-in in person?
Or -- could one simply go to the RCC in HNL and check-in there with kitty, thus staying airside?