Yes, it's possible
Here's what I use:
so = "!f() { c=`expr $1 - 1`; git filter-branch -f --msg-filter \"echo -n \\\"[SIGNED-OFF] \\\" && cat && echo \\\"\nSigned-off-by: Barton Chittenden <
barton@bywatersolutions.com>\\\"\" HEAD~$c^..; }; f"
... I'm not sure where that came from. It *may* have even come from an earlier version on the wiki... I think that the issue is that git bz attach will automatically obsolete patches that have the same subject line, whereas appending "[SIGNED-OFF]" may break that. Take that with a large grain of salt, test it yourself and maybe run it by the good QA folx.
--Barton