Timothy,
I have been granted overweight approvals for single engine aircraft, and have operated twins overweight under approvals from the manufacturers of the aircraft. I agree with the "calculate it" means of producing a "comfort zone" of data. Once done, I suggest that you request the issuance of a temporary flight permit for the purpose of testing it, and take some sandbags for a ride to validate the data. Load by extrapolating the C of G limit lines in the flight manual, and do your testing right in the middle of the now salled C of G envelope. Include slow flight and stalls at a safe altitude, in your testing. Make it a long duration flight so you burn enough fuel to get yourself back under normal gross before landing.
With a flight permit, it's legal, and with caution, it's safe. A "G" meter and restriction on the "G" of manuevers while overweight should satisfy structural concerns. Brake and tire capacity are a concern in the case of an aborted takeoff.
If you would like to PM me, I can send you something which might help support your request of the authorities for flight autority.
Cheers, Pilot DAR