This is one of the nonsensical (if that's a word) regulations that government's love to make. The car is not going to be shipped running in gear, duh. Anyway, the regulations were not made for cars with good ole American V-8 torque, assuming that it is an automatic and a v-8 car of course. I'm not too familiar with the Mustang brakes but the car has rear disc and the parking brake works by holding the piston/pad clamped against the rotor right? That being the case, disc tend to not hold torque as well as drums so I think it might be difficult to get the stock e-brake to hold. Most are cable adjusted so it is simply a matter of tightening up the cable to get maximum clamping ability. If that is not enough then you may have to resort to trickery like detuning the car so it makes less power. Maybe disable a plug or richen up the fuel. Extreme measures would mean something like a converter change or a line lock (on the rear wheels). Maybe easier to sell the car there and buy another one here.