Fun project. Carry on long enough and the riders will rotate through again and again, and become more familiar with you. I greatly enjoy Jay Dusard's, The North American Cowboy: A Portrait. 8x10 work on a little different scale, but an honest representation.
Once you become comfortable with your set-up and the variuos conditions you work under, it might be fun to consider full length portraits -- it's the pants that take the beating!
Actually, what sounds fun -- full-length portraits, but taken with the Rollie tilted at 45 degrees...like so...
8x10 RA4 print of my friend Bruce -- you might get strange looks like I got from him!