loaded the hotfix from above - ssg5ssg20.6.2.0r4-ef2 got the same validation error. It looks like my spare has the old signature key. I was able to find 2 of my ssg5's from branches and viewed the key and extracted the firmware off via tftp. I didn't think they would work and they didn't. They were revision 6.3.0r8 and r14 so I was in same situation with constant loop boot error.
It looks like I need a pre 6.3 image to be able to boot again and then be able to change the key OR a way to change the key with TFTP.
I have a dropbox if anyone feels compelled to share an old firmware.