I don't believe they'll add Envoy to all 757s. IIRC, the former-ATA 757s are the ones being retrofitted for TA service. The already US/HP 757s were going to stay in their primarily leisure routes and not be re-configured.

I agree that putting Envoy class on service to Hawaii would be an extremely smart move. Envoy class is inferior to most (if not all) other TA business class products, but it still has the slight edge over domestic F cabins which are generally what serve HI. "Fly to Hawaii in an International Business Cabin" would be a great win for US.