http://www.flyertalk.com/forum/briti...nes-avios.html
This thread should help you answer your questions.
Taxes will be minimal for a US domestic flight, but availability is uncertain, since Hawaii is a very popular use of miles. You could check to see what award availability is like on the Alaska Airlines website, but I couldn't comment what availability BA can see - you'd have to go through the main thread to see other people's experience.