Nick,
While I don’t own this album, it might interest you. Douglas Hedwig has recorded a CD called
The ART of the POSTHORN. He posted this information on the TPIN list last year. If you open the link you will see that he played with the Metropolitan Opera in New York for many years in addition to other ensembles. If you are looking for what the characteristic sound of a post horn is, I’m guessing that this would make for a very good introduction by a fantastic player. It doesn’t appear that the Mahler 3 solo is on the disk, but I’m guessing that you may find the inspiration for it in one of these recordings (sorry, I’m not knowledgeable to know which one specifically).
It looks like the disk is also available from Amazon:
The ART of the POSTHORN on Amazon
Hope this helps!