"nobody here does anything illegal".......
If you have an NFA firearm or other item, you are highly discouraged from bringing it to Hawaii due to the incompetent and and anti-rights legislature.
And now completely ignoring Hawaii law, you are required by federal law to notify the ATF if ANY NFA item crosses a state line. Period. Let's just say you live in Mesquite, NV just north of Vegas. You can't take your legally owned and Form 1'd SBR from NV to AZ which is 5 miles away. Or cross the bridge in Laughlin NV to AZ. That would be serious pound you in the patootie Fed time if you were caught.
Now back to the pertinent question... IF you built your SBR as a pistol first (and only you would know), take off the stock and put on a brace. POOF, it's a pistol again and not subject to the NFA. CAVEAT- notify the ATF you have done this to avoid any vacation time in club fed. Now, just take your LOWER ONLY to HPD to register it as a receiver or a pistol WITHOUT AN UPPER. DO NOT TAKE AN UPPER TO HPD IF YOU DO THIS. Or state its multi length, bolt action AND semi-auto. Because it can be any of those things based on the upper you put on. It can be a 6" single shot or a 16" semi depending on the day. Now if HPD says you cannot register it, ask them specifically if they are denying you the ability to register the firearm and why, get the officers name and badge number, time and date and send a letter to HPD with that info, copying your State rep and Senator, stating you attempted to register your firearm and were denied. You have now covered yourself legally.
Have a nice day