Did you catch this tidbit in Scott Cantrell's Dallas Morning News review of the concert?
"Muted trumpets lent savage snarls, but elsewhere, unmuted, gleamed over the orchestral mass. The mellow sound of the offstage posthorn, a valveless horn actually specified in the score, was supplied by Thomas Rolfs." [italics added for emphasis]
Of course—as most here know—the passage in question can't be played on a valveless instrument, posthorn or otherwise. Mahler's original manuscript specifies a military-style flugelhorn (valved), but he switched to a valved B-flat posthorn in the first published edition because he was afraid the typical orchestral player of the day wouldn't have access to a military flugelhorn.
James