Sumner,
Most Super Caps are rated for around 2.7V. Most simple Super-cap based keep-alive circuits use several caps rated for 2.7V connected in series to increase their operating voltage. The more complex keep-alives do use 2.7V rated caps with a DC/DC converter to charge and discharge them using 12V. These caps could be used directly in those Keep-alives or stacked in series in the simpler keep-alives.