Since the connection is under 12 hours, UA should have no issue accepting it for recheck as long as the inbound flight is either same ticket (with a baggage interline partner) or from a *A partner, even if it wasn't checked all the way thru initially. UA's policies allow for the delivering carrier to check it thru to the UA flight in those situations, so just because the delivering carrier doesn't do it (either by their policy, by error, or because it has to be collected in US anyways for customs), doesn't make the customer ineligible to check the bag for the 12 hour connection. Of course, it's always dependent on the agent you get, but as long as you're doing this immediately after leaving customs at the recheck location it really shouldn't be any issue, ideally if there's any pushback a simple "I just came in on this INTL flight, the carrier was having issues checking thru, but need to recheck" should alleviate it and have them accept the bag.