Russell's can answer better, but I don't think any other backrest will work with a Day-Long except theirs.
Can you share a photo of the material used? My first reaction was that vinyl would be better, but reading your message has me thinking.I had my Daylong built last year and have 12K miles on it with some trips over 3K miles. The seat is truly built for all day riding. I had the Grasshopper backrest before I had the seat built, and it bolted right back on in the same place. It even looks good with the Daylong.
I hope you saw the Sunbrella material they offer. I had that material put on my Voyager seat and I love it. It doesn't trap the heat like vinyl, and breaths a bit more than leather. A little harder to keep clean but it really adds to the comfort of the seat.
Best of luck with your new seat.