In the already, I'm sure, 500+ instruments of the Far Advanced, Incomparable, Platinum Layered with Titanium addition of GPO, soon to released in 2010, how about a Sarrusophone.
I was looking over the score to Ravel's Rapsodie Espagnole and found it called for one. I honestly have never heard of one. You know guitar players don't get around much in the woodwind sections of orchestras.
I suppose, after reading the above thread, that one might substitute the bass sax that will be in the big band addition coming to a computer near you real soon, but it just might be a little extra that might tweak someone's imagination to go for the heavyweight version of GOP.