A couple of us stopped to help a rider with an over-free freehub problem. We used multiple cable ties on it and it seemed to be working so we rode on and left him. We saw him hours later at the finish. The bodge had only lasted a few miles. In the end he hitched a lift back.
I wonder if raising the back wheel of the bike and dropping it while applying tension to the chain might knock the hub pawls back into position and lock them there? If so, then you'd have to keep the bike in a high gear and keep on pedalling, but it might work?