Magnesium isotopes would allow you to track the movement of magnesium.
Injection of the radioisotope 28Mg (half-life of 20.915 hours) into senescent cells may allow you to observe the direct effect of magnesium movement from senescent cells to other cells. If abscission were to occur, you could check abscised plant tissue. If you wanted to check if anabolism was also occurring, this may take more thought.
This is assuming senescents and abscission in the plant model takes under ~20 hours. Which I think is unlikely, however there are plenty of other 'mineral nutrient' and magnesium radioisotopes available.
Radioactive isotopes have the ability to emit radiation allowing them to be imaged. This is great when they are inorganic molecules, because plant tissue is (mostly) organic. These variables allow imaging to be highly effective at particular emissions.
A similar approach was done here (Pay-wall(another here with-out paywall)). However these studies added another step 'fractionation'. This may be a helpful idea, fractionation is a separation process to measure the abundance of Magnesium (they refer to it as Magnesium Purification). This is another reason to use an isotope, instead of imaging it may be useful for purification.