There's a fundamental difference between your main memory and your L3 in that the former is DRAM and the later is SRAM. In DRAM you have a charge hidden in a well behind a single transistor and it's designed to be stable for a while (the refresh interval) without anybody doing anything to it. SRAM doesn't have that static component at all, it's a set of 6 or 8 transistors which have two stable configurations when powered and which lose their state as fast as all the other logic in your chip as soon as the power is cut.

You can play with the temperature if you want, but the mechanisms that prevent unauthorized access in normal conditions will have their lifetimes extended or decreased as much as you change the lifetime of the data you're trying to access. And liquid nitrogen temperatures at least tend to make everything happen faster in CMOS circuitry. That's governed by a complex interaction between the effect of temperature on carrier density and carrier mobility, so I'm not sure that you couldn't slow things down with, say, liquid helium, but even then I'm not sure you're buying anything.