The refusal to permit overnight >4 hour layovers on that route is obnoxious, and something that I ran into on my recent MR. (The fare rules permit a stopover, typically for $50-$100, but that doesn't help you.) The conditions under which UA permits an overnight first-flight >4 hour layover are a mystery.

I think, though I'm not sure, that a free stopover may be permitted on r/t award travel to Hawaii (unlike within the 48 or to Alaska). In that case, as long as you're booking the r/t, you should be fine. Others can confirm.