> But try to convince someone that the name John ought to be written 「ジャン」 and you'll get the blankest stares.

That sounds like an American pronunciation variant with the 'o' shifting towards 'a'.

Common given names tend to have a fixed romanization though. See ENAMDICT for a bunch of Johns:

http://nihongo.monash.edu/cgi-bin/wwwjdic?1C

You can vary your romanization of your own name to what actually represents it most closely, but with a very common name like John it would be rather awkward.