Hello!
I don't know if there's any code around integrating atom and ide-backend. The new work-in-progress School of Haskell implementation directly uses ide-backend-client. It turned out to not be very hard to get it working with GHCJS and using websockets.
In particular, the ide-backend-common[1] package builds with GHCJS! This is the package which defines the protocol types and serialization instances.
I needed to modify ide-backend-client a bit to allow me to use it with websockets. Since atom is local, maybe ide-backend-client's default of using stdin / stdout is fine. However, it might be helpful that my ide-backend-client branch[2] makes it into a library and allows the caller to specify actions for sending / receiving messages. The current state of that branch also requires a special branch of ide-backend[3].
I'm glad you're interested in this! I'm happy to help out with it!
-Michael