I've uploaded HAMMER1 userspace code which compiles on Linux distros. https://github.com/kusumi/lh1
The fs itself doesn't exist on Linux, thus majority of commands that require HAMMER1 ioctls are actually not usable. In other words, newfs_hammer and hammer blockmap/checkmap/show/show-undo/strip/recover commands are usable. e.g. newfs_hammer'd image created on Linux is mountable on DragonFly. Notes: I haven't included mount_hammer at this point. Dropped hammer "info" command. Included DragonFly's fstyp which was originally ported from FreeBSD. Further work on kernel side is not planned at the moment.
