This might be the closest thing out there:
This is reading out the memory electronically, so it's not visible in the sense of a microscope watching the die.
You can't photograph that which has no image: Flash, EPROM, DRAM and SRAM shouldn't have any visible change whatsoever, and are off the list. Possibly, FeRAM is visible, because of the special material used in its construction; the same may be true for any memory made with ferromagnetic or phase-change materials.
You also can't use an electron microscope, for the problems highlighted above: electron bombardment easily flips states, corrupting the data. A low energy (< 10 eV?) electron microscope (I'm not sure which types can do this?) might sense the electric field at the surface, which would be an excellent way to tell. Or likewise, an atomic force microscope with a charged probe.
Tim