You can totally write XMonad as a Hydra config or extension. Honestly I expect someone to do it any day now, considering how fun it sounds.
If anyone tackles this, please submit it to https://github.com/sdegutis/hydra-ext
EDIT: I forgot that this would require the events feature, which is planned for the 1.1 release. So not all of XMonad is possible in Hydra yet, although most of it is.