i don't know that it would retain its value, but could you stamp something into the block in an adjacent area that would not be removed by machining? you could then take photos of the original stamp and your own id stamp IN THE SAME PHOTO. after decking, the original stamp would be gone, but your stamp would remain. by showing the "before" photo you could prove that it was an actual MO block.
just my uneducated .02
paul.