According to the US website, either US does not fly SEA-FCO, or does not offer anything other than economy class travel. Otherwise I'd be happy to try them, assuming you covered the difference in fare (and the extra little bit to do YVR-SEA-YVR.)

A quick check on Expedia suggests US Envoy class wouldn't be much more than CAD$10,000 more than an upgradeable AC H class ticket, which is only restricted in requiring a Saturday night (I'd be departing Saturday and not even arriving until Sunday, so not really a restriction) and a $100 change fee.