Thanks to the original post and all comments.
Finally I was able to upgrade the SSD firmware on my mid-2012 27" iMac. The iMac has its original HD and a Crucial m4 256G SSD installed, and is running OS X El Capitan.
The main problem I was having is that the firmware of the iMac itself had been upgraded, and it refused to boot from an external USB drive. Previous comments mentioned using the original HD instead, so I thought I could give that a try.
Another problem is that I couldn't download the refit-syslinux image file -- it's not accessible any more.
The basic idea of my solution is to use rEFInd as the boot loader for syslinux installed on the internal HD. I followed these steps:
dd if=/dev/rdisk2s1 of=/dev/rdisk1s1 bs=4m
Hope this helps!
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