The very useful bitbake-diffsigs script can do more than just diff signature data -- it can also dump the data from a single signature data file. I think this functionality should ideally be made obvious from a naming perspective. E.g. a symlink from bitbake-dumpsig to bitbake-diffsigs, or a wrapper script to avoid storing a symlink in git, or a standalone script utilizing the same functionality from the siggen module.
Fixed in master: http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=70a03756d4c9dc93402e2db3d71f515de91b6fa0