I have the same problem. You should be able to support all defined character codes! I maintain a considerable web site in Honolulu, and can use Hawaiian diacriticals with no problem, but Constant Contact simply won't accept a ū and I see no reason you shouldn't be able to do this!
You can post a new message in the Community or find us on Twitter Mon-Fri 8am-8pm ET, Sat 10am-8pm ET, & Sun 10am-6pm ET. We've got real people waiting to help you out. Click below to start a conversation!