Basic dm verity userspace support is available in the upstream lvm2 respository. This should be extended to use libdevmapper and packaged for Fedora (as an lvm2 subpackage).
Based on or discussion , I'll provide veritysetup in cryptsetup package (with all needed fixes mentioned above). (Let see if it works, not problem to switch to another implementation later) For now, the tool in in separate branch http://code.google.com/p/cryptsetup/source/browse/?name=veritysetup
New cryptsetup subpackage called "veritysetup" is on the way to rawhide. http://koji.fedoraproject.org/koji/taskinfo?taskID=4150743 It uses cryptsetup-libs and libdevmapper usual way.