Keep in mind that in many applications a capacitor is used to store an analog voltage for short periods of time (ex. peak detector, track and hold for ADC, etc).
It may be possible to build a circuit to hold an analog voltage for a while using a capacitor, but you would have many problems b/c you would have charge and discharge cycles on the capacitor and every error on a charge/discharge cycle would be cummulative and eventually, the capacitor would be discharged or charged to the supply voltage.
If a device exists, it would be interesting indeed.