Can I use a hole saw to cut some new holes in the back of an outdoor breaker panel? Turns out the knockouts provided happen to be below the fire blocks in my walls... Anyway, with a small amount of luck and measuring, drilling new knockouts in the top might provide some access above the fire blocks.
Alternatively, I could always go back to my stellar idea about external conduit, but with THHN wire going in the conduit and many parallel conduits (like 6 per box to max out at 4 conductors per conduit). It'd be a whole heck of a lot easier than running the wire through the walls (I guess).