Naw, there's no real mechanism to withdraw from the contract for that reason, especially if the offending DC Land is in a separate park (the new one).
I'd keep an eye on it in a few years; at some point the Warner-Six Flags agreements will come up to renegotiation; there's at least a small chance that Universal (or who knows maybe Legoland) could talk to them about building out a Florida exemption from the Six Flag's US DC rights.
I think it's worth Universal setting aside some expansion land for specifically in the new park. Set aside a large land's worth of acreage and show it to Warner and tell them you'd build them Metropolis/Gotham etc. I think they'd bite on that if it's a significant enough opportunity (which it would be).
Heck, Warner would make more money on just a DC Land in the new park than they probably do from all their Six Flags' US DC rights... That alone is a reason for carving Florida out in the next agreement and working with Universal.