My guess being steel the specialist company repairing it will do MT or MPI testing on it as much quicker as gives instant results.
You are correct in that normally what you can see visually is only a small part of the picture.
PT dye you put on, wait 15 to 30 mins for it to soak in, clean it, dry it and then apply a developer. Very useful to have a 3 can kit in the workshop. If your eyes are good you can soak the part in WD40, clean it and lightly dust it with powder as the oil will stain the dust and was how the principle was found in the first place
The only problem with castings, especially old ones was that the material is not homogenous so you can pick up indications in the surface that are from manufacture. Another reason why MT testing is good as when you find something you tend to give it a quick grind to see if its got any depth instead of a scab. MT just needs a magnet and more ink, preferably fluorescent.
Both these NDT techniques are fairly easy and cheap. http://hmvf.co.uk/forumvb/showthread.php?16151-Ndt
But all in all I hope its all a small thing and easily repaired so you can continue with your great thread.