Nice blog Retrogrouch.
My wife and I did a little riding on Maui on our Bromptons at the end of April earlier this year. I had read mixed info about riding on the island but wanted to give it a try. Because of the wind and hills some seem to think that the island is only suited to the serious rider wanting a challenge or training in a beautiful locale. We stayed two nights in Wailea hotel, then four nights at Kaanapali cottage, then two more nights in Wailea. Our original plan was to forego a rental car and just use the Bromptons for transport. However an old college buddy and his wife were able to join us so we ended up sharing a rental car and almost did not bring the bikes, since our friends are not riders.
I scoped out the highways between the various parts of the island for future trips and they seemed as you say - high traffic, highway speeds with wide shoulders so not the most pleasant riding but certainly adequate for moving from one area to another. For those thinking of travelling by bike the routes from the airport to Wailea or Kaanapali are relatively flat but winds are common. We did some short, localized rides like you mentioned - Kaanapali to Lahaina, Wailea to Kihei, etc. During a Brompton shopping cart trip through the Lahaina Safeway we discovered that we could fit a six bottle wine carrier into the T-bag, a handy discovery. Although we had a great time with our friends I wished that we had ridden more. We have talked of making another trip in the future and doing without the rental car.