Next time I travel back to mainland, I intend to take back a few firearms that I have legally registered here.  Is there any requirement to, or is it a good idea to, notify the good folks at the MPD registration office here?

Mahalo for your input

State law says any firearm permanently leaving the state must notify the police. "Permanent" is the key.  So if you plan on coming back with them after 20 years, that's not permanently leaving the state. IMO more paperwork for MPD to mess up and hassle you over.
 
There is no way to un-register a firearm.

It's not just a good idea.  It's now the law:

(e)  Every person who permanently moves firearms out of the State shall contact and
notify the county police department in the county where the firearms are registered about
the removal of the firearms within five days of the removal from the State. 

Any person who fails to timely notify the appropriate police department shall be subject to
a civil penalty of $100 per firearm.

https://www.capitol.hawaii.gov/hrscurrent/Vol03_Ch0121-0200D/HRS0134/HRS_0134-0003.htm

If it were me, I'd make copies of the registration forms for each of my firearms leaving the state.  I'd include those copies along with a notification letter listing those firearms and stating the date the firearms will be leaving. 

Whether you deliver the letter in person to make sure everything is taken care of, or mail it to the police department using a return receipt service, you should be covered.  Don't just rely on USPS Priority Mail tracking.  You should get a signed return receipt to cover yourself when mailing anything that's required by law.  It's proof you not only sent it, but that the recipient physically signed for it.  It avoids the whole "If we don't have it, you must not have sent it" situation.

Hey guys, thanks for the input.

I ended up calling the registration desk here on Maui, and they said to send them an email with attached scans of the registration forms.

That will provide me a permanent record and I can complete it in a few minutes. All good!