Glycerol sadly has the tendency to supercool,so once all of your solution melts you will lose all of your seed crystals and then you have to drop down to a really low temperature before it will get back to a solid.
@juul, note that glycerine is denser then water. A solution of 95% glycerine and 5% water at around 5 degC is about 1.25 g/mL, so by volume it would closer to 3/4 of the capacity. Source.
Of course water won't go through a phase change above 0 degC, so all told the glycerine might prove to be a better temperature buffer if you can get it to actually freeze. Also, glycerine is a poor conductor, so if placed along the walls where the cooling elements are located you could reduce the temperature fluctuations that occur when the unit turns on.