Absolutely in hand carried luggage. The thing with Customs is this: as you may recall the declaration form requires you to list items that will remain in the U.S. Strictly speaking, this means you would not have to put the engagement ring on the form. However it could cause an issue if they do find the ring and ask why it isn't listed on the declaration. You'd have to explain it, and then of course the surprise has been blown courtesy of our wondrous Customs & Border Protection service.
Please ask your question on other forums as, frankly, we have little expertise around here for questions about non-U.S. citizens and Customs. Perhaps our Canadian members can help shed some light.