Just thinking about this and it occurred to me that if one can come up with a mathematically chaotic function that can be expressed within the limitations of finite voltage rails and the operations easily obtainable with analogue electronics (+, -, fixed multiplication and division, derivatives, integrals, logs and anti-logs) then one could create a pretty convincing 'random' display. I'll leave it to someone who's looked at chaos theory more recently than me to come up with a suggestion.