To follow up on this, I was able to make the migration a small step at at time.
The tool seems very hardwired on the name and path, so I started by copying the exact generic example panel from the CS6 SDK.
Then I carefully copied bit by bit of the old properties.xml file into the corresponding one in the generic panel.
I found the tool failed on a particular line; I think it may have been an unescaped "&" in a quoted label field.
Finally I got a functional copy running under CS6.
That file had some hidden dependencies that I found confusing, and it did not seem to have made a complete copy of the XMP schema for some custom tags we defined, but the syntax was nearly the same so then I manually updated the running version bit by bit until all the definitions were in the new panels folder. Then I renamed it to something useful.