Thanks for the replies. A couple of things though. There is nothing that says you have to use flex to connect a transformer. In fact, is not allowed to enclose the GEC.
Fault current is not going to stop at the panel. It is going to flow back to the transformer regardless of where the bonding jumper is. Under a fault condition most of the current is going to flow on the metal raceway. For this reason I think it is a good idea to put ground bushings at the transformer and panel but I don’t think it required.
Also, for circuits over 250V to ground the requirement to use ground bushings only applies when concentric or eccentric KO’s are encountered. Transformers usually do not have these.
I looked through some more literature and still don’t see that they are required do to the nature of the conductors involved. Maybe it’s one of those things you learn when starting out and eventually find out it’s not true. I should write a book of all the things people have said to me 'are in the code' that simply are not. Might be interesting.