h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

There is one more: Borodin's Polovtsian Dances from Prince Igor.

The variation concerning the man on the tricycle (Sorry, I can't give his identity or the variation number at the moment) has 16 drum beats that are taken from the Polovtsian Dances. The rhythm is
(Q Q Q R Q R) 4 times, where Q denotes quaver beat and R denotes quaver rest, with a crescendo spanning the 16 beats. You must be familiar with it in both works!

So we have eight identified composers in the variations.

There is still the question of whether the finale contains a quotation from another Elgar work. (Surely it must?)