I'm not convinced it is Bix either, but there is something there of the feeling his solos have that keeps me from being sure it is not.
One aspect which hasn't been mentioned in this particular thread (but was brought up earlier) is that if Bix was there for the session, he was "wildcatting," in violation of his Whiteman contract, and might not have wanted to sound too much like himself, a ploy which might have made his attempts sound a bit tentative. It's a remote possibility, but one that could be posited, that the members of the band denied Bix's presence because they didn't want to damage his reputation even posthumously--or get themselves in trouble with their union.
I am aware that all these suppositions sound like fictionalizing!
The solo seems to me not to have been miked very closely either. Does it seem to anyone else that the mystery soloist is further from the microphone than Spanier was? The vocalist, Muggsy, the accordionist in Take 3, and the trombonist who plays the bridge seem front and center, but the second cornetist seems to be back with the rest of the boys.