BB, I believe it has *always* been at the IA's discretion
to make major alterations. The rub is that major alterations
require approved data.

Well, quite. If the alteration has approved data (eg. an STC) then the
IA can sign the 337, and he then sends it straight to OK City. But if
he merely has acceptable data, the most common route open to him is a
Field Approval whereby he sends that data with a 337 to the FSDO, and
gets them to turn that into approved data by stamping it and signing
the magic box. The IA can then sign the form after completion, and send
it off to OK City for filing. But that still means that the FSDO is
involved in the Field Approval process. Do you still assert the Field
Approvals do not involve a FSDO and go straight to OK City???