I'll have to agree with Dave on this one. Miller was very particular about the sound of his band, and it seems unlikely that he would be interested in an arranger who reached his musical peak a decade earlier.
With regard to Bobby Hackett, I've always considered his beautiful cornet solo on Glenn's "A String Of Pearls" to be an unabashed tribute to Bix. So I think it's unfair to say that Glenn thought Bix's musical influences were no longer relevant.