As time passes more slowly in a region of space close to the source of a gravitational field, shouldn't the moon, which always has one side facing towards the earth, have a higher fraction of radioactive isotopes on that side than on the far side? Could the fraction be used to determine the time tidal locking occured?
Also, shouldn't the fraction of radioactive isotopes be different in the earth depending on how deep you dig? Gravitation drops off with $r^2$, whereas the gravitationally relevant mass increases with $r^3$ (only the mass of the earth's sphere lying below the radioactive atom results in gravitation due to Gauss' law) , so outer layers should have more time dilation and therefore a higher fraction of radioactive isotopes. Can this be observed?