The outside of the block would be in a direct line to the anode(s), so would be line of sight. Unless you can get an anode inside the water passages, then the current would have to take an indirect path. That would not be line of sight. The current would still flow through the electrolyte, but it would have to travel a longer path with more resistance.

When I was doing the lathe chuck, the center hole was taking a long time to clean up. I suspended an additional anode in the center of the hole and it cleaned right up. I'm not saying it won't work in indirect locations, it will just take longer.