If you can't flex, try a hell lot of isometrics. I'm far from an expert, and have never tried it to be honest, but what do most BB'ers (aka hypertrophy freaks) have in common? They train with a significant time under tension.
Say, 10 reps, 2 second each rep. That would equal 20s TUT. Do something for obliques and rectus abdominus (AKA abs).
Try (one-hand) farmer walks, suitcase deadlifts, pallof presses, landmine twists, etc for obliques. Focus on higher reps for more musclr growth (like 20-30 sec TUT).
Try Rkc planks, body saws, etc the same way.
Hope it helps.
Btw I saw a vid of a guy lying on the ground, putting a kb on his belly and pushing his belly out against it for reps. Maybe an idea for improved bracing?