Would it be legal/practical for a US citizen/resident to register in Hawaii to obtain the kama'aina discounts?

Just to add to Zach Lipton's excellent answer, I want to bring up some other points if you can successfully argue that you've become a Hawaiian resident:

• Your existing driver's license will cease to be valid soon. Hawaii has time limits, set by county, on how quickly you must change. (Hawaii requires you to have a social security number to get a driver's license, so this won't even be an option for foreign nationals unless formally immigrating.) Driving without a valid driver's license will:
  • Make you subject to penalties for driving without a valid driver's license
  • Make any auto insurance you have in another jurisdiction be potentially invalid
• There may be health insurance consequences. For example, if you are covered by a Canadian province's health care program, you will cease eligibility if you become a foreign resident. This will invalidate any travel medical insurance you bought in Canada, which is predicated on the validity of the underlying provincial health care insurance. You'd need to buy US health insurance, which is complex.
• Becoming a resident would cause any property insurance you have to stop covering you before long. For example, most Canadian habitational insurance allows you 30 days of coverage while you relocate within Canada. Becoming a foreign resident would be prima facie evidence that you have relocated outside Canada, which would invalidate your property insurance immediately.

It seems a lot of problems to endure, just to save a few dollars on some admission tickets or hotel rooms.