Unless I'm not understanding, inside your old massive cold plates is tubing which is cast into the plate. The plate really functions as a "flywheel" to use it's mass to absorb heat. In order to cool down the plate, they surrounded (in some cases) the tubes which have refrigerant (today, say 134r, in your case maybe something else?) with a solution that was contained within the "plate". The solution was then encapulated within the (block-like) plate. Is that what you have, Neil?
If so, then you can seal-up the leak with a number of things whose expansion coefficient is similar to the plate material -- or a sealant which will operate to gap the holes or split. The key, I would suspect is that you need to refill the fluid that surrounds the coils or you'll have very inefficient transfer of heat between the plates and the refrigerant. Is there a "plug" or some other means of replenishing the fluid? Do you have the fluid to replace it? Again, you don't want water, per se, it's more of an antifreeze solution. I'd be careful with toxic materials or solvents (like alcohol) which function like a anti-freeze. (The freeze-thaw expansion would cause problems with stress cracks in you plate?)