Tool for randomized signatures
About the only luck I've had, other than writing scripts myself, is using SignatureProfiler.  Note that it's not free ($12).

It's mostly overkill for what I need, which is just the random signatures, so I've implemented a few little scripts to get this working on my own machine - Random Signatures for OSX

Note, however, that something seems to have changed in 10.10 that is now preventing Mail from updating the signature when the signature file itself is updated (until I manually trigger it, or restart mail).  They must have some kind of caching in mail now for the sig files so that it's not read in every time.