A couple months ago, I went thru a search for the same thing. The only backrest I found that would work was the protac (www.protac.net). It mounts on each side to the side rail. I didn't like the look, and it would not move with the air seat.
I even tried a couple of ways to make a prototype that would mount between the seat pan and the metal seat cross frame piece, and curl up behind the seat. I could not come up with anything that worked, and that would still allow a passenger seat.
I just bought a mustang one piece off ebay that I will put on here soon. I really wanted a backrest and a comfortable passenger setup, so I have given up on the police air seat. It's kind of bitter sweet. I do like that seat.
Yes, I have an 03 FLHTPI. Picked it up 9/1/05. I've had the fairing off several times. Not too much to it once you do it a time or two. No radio, but I do have speakers that I put in the fairing.
But, right now, everything is off the front end. I am converting it to a Road Glide (fixed fairing). Just ordered the parts tonight. It'll be a project, but it will be fun. I am going to sell all of the batwing parts/brackets/lights etc.
Good luck with the backrest. I just mounted my mustang tonight and sat on it. Feels good. I will get the mustang backrest for it at some point.