Nick,
a real post horn has no valves and can not be used for the solo. In Germany, we have a valved version called "Fürst Pless" (
http://www.melton.de/Jagd/R1174.jpg). It is tightly coiled and wrapped in green leather and has rotary valves. It has no redeeming social or musical value.
I think the horn used is insignificant, it just needs to sound like it is coming from FAR AWAY. The further "off stage" you are, the more helpful a trumpet would be. If you can't get far away, the flugelhorn or cornet have less overtones and sound more remote. I would change as little as possible, too much margin for error! An error could completely destroy the mood..........