Back at home, and found the reference to powder burn rate and recoil in Thomas' book. From the chapter entitled "Searchlight on Recoil" in "Gough Thomas' Gun Book":

"On the other hand, we have I.M.I.'s prolonged researches which I have described in "Shotguns and Cartridges", in the course of which it was found that a team of shooters, firing many thousands of cartridges loaded to give the same velocity to the same shot charge, and therefore developing the same DYNAMIC recoil, unanimously voted that the variety giving the least SENSIBLE recoil were those that, unknown to them, had been loaded with the fastest-burning powder." (Emphasis the author's.)

Thomas gives a fairly long "personal explanation"--his own theory as to why this is. In a nutshell, he feels that with fast-burning powders, "the pressure on the shoulder is becoming too fleeting for the nervous system fully to record it."

Interestingly enough, at least based on the Alliant catalog I have, it would appear that Alliant Powder pretty much agrees with Thomas. They list their powders by burn rate, from fastest to slowest. Bullseye and Red Dot are #1 and 2, respectively. #3 is American Select, #4 is Green Dot. Alliant refers to the "less felt recoil" with American Select, and "lower felt recoil" with Green Dot. Since the only really commonly-used shotgun powder that's faster is Red Dot, and since several Alliant products are slower (Unique, Herco, Blue Dot), and since they don't tout ANY of those slower powders for recoil reduction, they seem to be telling us that relatively fast-burning powders are the ones to go to, if you're looking for less recoil.