The one workaround is a contract Verizon phone, which has an unlocked sim slot. Pay the ETF and you technically have an unlocked GSM phone.
It appears that the T-Mobile model is not hardware locked at all. . . just pull the SIM card and ignore the signed contract. You are only required to have a 44 day obligation... Of course you've paid the full retail for a T-mobile iPhone 6 in the first place, plus connection fee and 44 days connect time.