I don't think they have a seatstay specific mount but I imagine with some ingenuity you could take the original mount and zip tie it around (maybe use some shims if needed). I would get in touch with Topeak and see if they actually make a mount or have/will consider one. They are pretty big into modularity and I am sure they might consider it.

I have mine mounted on the seat tube because I have a ton of space between seat tube and my fender/rear wheel. I occasionally move the pump to my Cilo and that is done with a side mount as you have done.