12v capacitor / UPS system
If you can reduce your requirement to the situation of the car engine being started and the battery voltage being significantly lower than 12 volts then try using a boost regulator set to produce 15 volts followed by a 12 volt linear regulator to exclusively power the cameras.

It will probably be cheaper, smaller and more reliable than using a large bank of capacitors. This sepic controller will possibly do the job just as easily: -



It works down to an input voltage of 5V (this may be ok during turning the starter motors) and directly produces 12V without a linear regulator. Is 1A sufficient for the camera system?

No real worries about the input from the battery/charge system on this - the LT device is rated up to 40V on the input but some spike filtering from a series inductor and parallel capacitor ought to be used (or even a LDO linear voltage regulator set for 15V just to take away the "nasties").