Coinbase can help you retrieve it, even though they might not know it or they might not want to.
A USDT address is the same as a Bitcoin address. If you import the private key for the Bitcoin address into a USDT wallet, you will be able to sent the USDT. Sorry, I can't give you details because I am unfamiliar with Omni (aka Mastercoin).
The mistake you made is not uncommon. Here is a recent reddit post describing a similar mistake:
https://www.reddit.com/r/btc/comments/4 ... _currency/