A FlyerTalk Posting Legend
Join Date: Jul 2002
Location: MCI
Programs: AA Gold 1MM, AS MVP, UA Silver, WN A-List, Marriott LT Titanium, HH Diamond
Posts: 53,006
I have done stopovers on Hawaii awards on both AA and UA. Not sure where you intend to go though: AFAIK the stopover has to be along a valid routing to Hawaii.
The good news is that both of those airlines have quite a few valid routings to Hawaii. They fly from their mini-hubs on the West Coast to Hawaii, and they fly to both HNL and other islands. If you have some date flexibility, you can pick from a lot of interesting stopover options.
I have no working knowledge of NW/CO.