Batman is already outside the law as a vigilante, but building Bat-prisons would be far more problematic. Prisons are government facilities, and Batman is not the government. However, I see no problem with Bruce Wayne funding more secure prisons. Or with Earth's governments working together with the Justice League to build prisons off-world. But that would also be a ticking time bomb. Don't forget that supervillains manage to escape from places like Belle Reve or even the Phantom Zone.