I believe that it is more a conformation thing than a gait thing. None of mine move hollow backed, my old mare may not buy heck, she is 27 years old! Frog can probably enlighten you more on that since she has had/bred them a little longer than me.
Frog, how did your ride go? We went to Bucks Pocket in Guntersville Sunday, it was an OK ride. Mostly old fire roads. I will post pictures shortly.