Big Brother?
Unless this is another "between the lines" interpetation of the law, there is nothing in the books pertaining to license plate covers, except that they may not be used for the purpose of deterring law enforcement. Meaning that if they are clear, and do not obstruct the view of the license plate, or alter its appearences, are o.k. Bottom line is, though, its up to the discretion of the inspector. There are some lenient inspection stations, as we all know, but we shouldn't publish the names here or in any public forum, lest they no longer be lenient.
As for the legality of other parts, if you purchase from a reputable local retailer, i.e., Rons, etc., They will tell you if what you are about to purchase is or is not legal for "on Highway or street use" as they work side by side to be sure what they sell is legal. Also, check the packaging, or if mail ordering, the disclaimer or parts description, this will also tell you for on highway use. If you want to be a real pain in the butt, you may call the recon station and ask them if what you are planning to purchase is legal, under Hawaii law.
BTW the guy at recon with the cowboy hat is Kevin Brown, and, although people say he is kind of a hard ass, he is probably also the most helpful when calling for information/help. Whatever you do, do not give these guys a bad time. They are doing the job they are paid to do, making sure the vehicles on our streets safe. If you disagree with the law, see you representative.