Two ideas: 1) have made canti levers that are sized up, both higher and with substantially longer arms. Run them with long-pull V-brake levers. Granted, not easy, requiring both design work and fabrication. And 2) are there drop-bolts for Mafac Racer style centerpulls. Seems to me you could make a long drop-bolt that had a rear leg that came back, then up to the rear of the fork crown to stiffen the assembly.
A challenge, but not un-solveable. Easy? Maybe not. If I had your bike, I just might try. I am after all the guy who set his old custom up to ride fix gear with three compete sets of drivetrains for real mountain days, 46-12, 44-17 and 36-21 (and can go down to 36-24). A lot of work, both design and execution and it runs really well! If you figure this out, you could be on to something.
Ben