Currently bitbake-diffsigs with the -t option isn't as useful as it was before we started to delete old stamps and their siginfo files. It would be good if we could move these files to some other directory before wiping the stamp and then have the siginfo search code in OE-Core look there as well.
This has now been fixed by Richard in master: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=525cc08cc556277c69a1b2bdf027df9cd74de0cb