SQ doesn’t offer check in in T1. However it is common enough now for flights to depart from the gates in T1 adjacent to T3 (eg. C1, C20). Can use sky train to get there and time to reach these gates from the lounges is only marginally more than reaching B9 and B10. Not necessarily US flights, I’ve seen flights to various destinations go from these gates. Last two I saw were ADL and CPH.