When I asked HPD, their response is that since you cannot legally possess an AR pistol with detachable magazines, AR magazines are not pistol magazines.
Certainly not a defense in court, but as a practical matter they do not seem interested in enforcing at this time, and I imagine as long as nobody does something stupid to make them do so, it will not be an issue, but that's pure conjecture on my part.
Anyway, given how poorly written HRS 134-8 is, their stance makes sense. If the argument is that since it says "pistol" and does not specify "pistol otherwise legal to possess in Hawaii", I assume I could then manufacture a bunch of pistols designed to take all the major rifle magazines (not hard in a free state), and essentially ban all such rifle magazines in Hawaii - since there is now a pistol out there that is capable of being used with said magazines.
Which would be totally absurd and I doubt would stand up in court.