Hi,
I assume that when you want to 'read' the stored value that you must power up anyway.
The way this used to be done back in the age of some of the first computers before floppies and hard drives came along was to use a magnetic memory core. Each cell was made from a small toroid core with two wires through it. When current was put through the two wires in the correct polarity, the core would magnetize in one direction, with current opposite, the core would magnetize in the other direction. This of course provided for a mechanism in which to store ones and zeros (digital data).
To extract the data a sense amplifier was used to detect the change in magnetic state when a pulse was applied. Assuming you have to power up to read the data anyway this wasnt a problem.
Another method which might be simpler using today's technology might be to use an easily magnetized metal with a coil of wire wrapped around it. You could drive the coil with transistors. With current in one direction that would magnetize with one polarity and with current in the other direction the opposite polarity, which provides two states which can be called 'one' and 'zero'.
To read the 'data' (ie magnetic state) you could use a linear hall effect device. The hall effect device will provide a voltage that is greater than 2.5v for say a logic '1', and a voltage that is less than 2.5v for a logic '0'. Thus depending on which direction the current was flowing to the coil prior to power down the stored state can be either a logic '1' or a logic '0'.
This would hold the value for a long time also.
Some CMOS circuits take very little power though, so a small battery could keep it powered up for a long time, thus saving the logic state for a long time. The read circuit would be powered up only when it is desired to read the data, so the battery should last a very long time. This is the way PC computer mother boards maintain some CMOS data and real time clock.
Floating gates are a bit temperamental though, and can change state by themselves with a mere change in temperature. I'd stay away from that idea without investigating the pros and cons thoroughly.