Regardless if pistol or long gun, the SELLER has 48 hours to notify the department that issued the permit to acquire the transfer has taken place. Showing up at the same time to pick up the permit to acquire (if not already in hand, re: long guns) and register the gun is just a convenient way to do everything at once. It also makes transferring the firearm less of a risk, as there are only a few public places to which you can legally transport a firearm. The police department is one of them.
http://www.capitol.hawaii.gov/hrscurrent/Vol03_Ch0121-0200D/HRS0134/HRS_0134-0002.htm

That's one way to read it...but as with most HRS 134, parts are so poorly written/worded that people try to interpret the who and what of it all. The "person who is transferring title" can be read as transferor or transferee. Try inserting either of the better and more definitive words in place of the quoted part, then it can go either way.
I completely understand some wanting to err on the side of caution, but doing a transfer in person and notification by seller is not required. Here's a quick break down...
Both sections begin with:
In all cases where a ..... is acquired from another person...
That unquestionably refers to the buyer and so does the entire section.
Also, from the pistols and revolvers section:
The person who is transferring title to the firearm shall sign the permit in ink and cause the permit to be delivered or sent by registered mail to the issuing authority within forty-eight hours after transferring the firearm.
The seller does not take possession any permit in a transaction, so he can not be responsible for delivering it to the issuing authority.
It's not a play on words or some personal interpretation. Fact is notification of transfer is completed when the buyer registers the firearm (as required by law). There is no "unregistering" a firearm for the seller...everything you've ever owned and registered will always be on file.