I went ahead and added the necessary udev rule to the kmonad package. The revised patch is attached. I've tested it and confirmed that if you add your user to the input group, and include kmonad in your system udev rules [1], you can run kmonad as an unprivileged user.

[1]: https://guix.gnu.org/manual/en/html_node/Base-Services.html

-- 
Alex Griffin