Barry, that's one thing that's had me wondering for a long time, but thus far, I've seen nor heard of anything.
I DO, however, have a copy of Karl Böhm's memoirs, but it is still packed away in a box from my move. Things are slowly falling into place for me in getting things musical unpacked, and I'll try to remember checking in that book to see if Böhm conducted any other Mahler performances. If I see anything, I'll let you know.
Wade