The airline transport pilot was landing at an off-airport site in a tailwheel-equipped airplane during a Title 14, CFR Part 91 personal flight. The pilot reported that after landing, the airplane veered to the left, and he applied full right rudder in an attempt to correct the veer. The airplane subsequently went off the left side of the site, entered a ditch that contained large rocks, and nosed over. The airplane sustained structural damage to both left wing lift struts. The pilot noted that there were no preaccident mechanical anomalies with the airplane.