Sell those trees and get ones that do well in your location. I grow a few tropicals in a temperate climate (still downsizing that collection), but at least with those they just sit under lights inside when it gets too cold. Trying to trick a plant into winter dormancy is a much much bigger battle that isn't worth the time. Somebody on here was adamant about putting a maple in a fridge for winter and it died back all over with desiccated branches.
I don't want to be that guy, but somebody had to say it. I'm sorry. You've got a ton of available species that are great for bonsai.
You just have to make sure the trees being vernalized in the refrigerator do not dry out (the soil), just like if they are being kept outside. Deciduous trees do not need any light when leafless. Allow the tree to have a full growing season outside, then simply place it in the refrigerator. I cut off all the leaves before doing so, but this is not necessary, as they will fall off after 3-6 weeks in the fridge and go dormant anyways. "Tricking" a tree into going dormant is as simple as setting it into the fridge after a growing season of appropriate length, and of course, watering it when necessary, like you would at any time.
Bonsaiguy_2012, if you want to grow a few maples, go for it (don't get rid of the ones you have) and find access to an adequately sized refrigeration unit. It is really not such a bizarre concept in practice as it must seem to a lot of people.
I think having a large collection of temperate trees in a tropical/warm subtropical climate would be a burden, but having a few is really no big deal, since refrigeration really does work. If one has deep pockets, a large walk in fridge could easily accommodate a very nice temperate deciduous bonsai collection. But then, I am a gardener (and amateur bonsai artist) who does not settle for just what will grow in my location without extensive care and artificial climate control. I grow palms that are too cold sensitive to survive an average winter in Miami and trees that would die after only a year without 4 months in the refrigerator. I enjoy the challenge and the variety.
Those trees under cover at 4,000 ft in the Hawaiian rainforest look great! I have seen other photos of thriving temperate deciduous tree species in higher altitude locations of Hawaii as well. Acer skutchii, a sub-species of sugar maple native to the mountainous areas of Mexico and Guatemala might be a good one to try. Also, I am now under the impression that the Taiwanese subspecies of trident maple should do well in areas with very few chill hours.
