It would be great if I could use a sub-classed Decimal during parsing but since it doesn't appear to be a way to serialize the result using the "json" package I'm probably stuck with the current "99%" solution.

I have solved this in Java and JavaScript by writing my own JSON stuff
http://webpki.org/papers/keygen2/doc/org/webpki/json/package-summary.html
but that method obviously doesn't scale and I'm a real n00b when it comes to Python although it was more fun than I had expected :-)

A minor patch addressing serialization of Decimal would probably do fine (after sub-classing) and would be generally useful.