Problem finally solved--I've got the HD Native card working with the OWC Chassis in TB2.
The biggest issue was that startup would hang on the restart after "allowing" the extension to load. I had to run Disk Utility on the startup drive in Recovery Mode, and that did the trick. I thought I had tried it before, but I was trying so much different things, that maybe I overlooked it.
Another possible cause was that there is a second extensions folder inside "Systems". That contained some way older extensions, one also named "DigiDal" from 2012, and another even older kext for the Digi Eleven box. Since I deleted those, trashed PT prefs, and went through the thing with the startup all at the same time, I'm actually not sure which of those was really responsible for the success. Anyway, this document got me going down the correct path, I think....
http://avid.force.com/pkb/articles/e...-Tools-upgrade
I actually didn't know there was a second extensions folder, so that was a revelation. Anyway thanks everybody for your responses--