I doubt that Don would have the classical training to read sheet music in general let alone the chops to play Zappa's music. That being said, I hope he was there and that they allowed him a solo or something....
Also, considering his erratic behavior (on and offstage) during the Bongo Fury tour-which Zappa did take an offense to, I doubt that Zappa would have trusted him to play in his orchestra (considering how tight they would need to be and how expensive it was to organize those concerts-6 figures we're looking at).
But like I said, I hope I'm wrong and he was there to perform...

Hi - I was at the Abnuceals
Royce Hall show, and Don did
blow some sax - it was during
the final song, the end section
of Strictly Genteel, and Don
just wailed in his usual style...
Of course, he was not in the
orchestra, he just came out in
the end... I saw him after the
show, outside, and he seemed to
be in a good mood...

Wow, I recant. So did he play during "SG" or was it for an encore? If so, was he playing with the band or was it that the band stopped and let him go nuts?

Don played his wild sax over the chords at the end of S.G. The orchestra played those simple chords repeatedly, as a vamp...
This was all recorded by Zappa,
with a recording studio truck at
the back of Royce Hall...
Also, an audience recording exists...

Both of these wonderful concerts are HERE!
http://www.zappateers.com/bb/viewtopic.php?t=3470
http://www.zappateers.com/bb/viewtopic.php?t=3459