I still rather like that symbol. The 'wrap around' is fairly descriptive, if only to remind you that the outer can is at negative potential and a potential accidental contact hazard in negative rail supplies. It could be confused with an outer foil indication if somebody forgot the '+' symbol and value though.
The idea that it harks back to the Leyden jar is intriguing, there's probably more than a grain of truth in that. Leyden jars were shown diagrammatically at some point (trying to think back to some of my old books).