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There are no free stopovers on UA's one-way awards, only on their roundtrip awards.
If you can find SAAver award seats on AA/AS nonstops from JFK/EWR to LAX/PHX/SAN/SEA/SFO, and from there to HNL, then you can get that redemption for 25,000 BA Avios (12,500 per flight), with or without a stopover on the West Coast. AA award flights can be booked directly on ba.com; AS award flights require a call to BAEC.
You can search aa.com for SAAver award seats on both AA and AS; if you find the seats you need, you can then book them via BA.