If I want to learn how to interface with some garmin
gps from my pc, I can google for the specification
of the garmin interace and slap together some C code
to upload a trace or otherwise fiddle with my logger
unit.

And I can do the same by opening an approved FR and fiddling
with the GPS streams from the engine, and I can do that at
home without an OO watching and get a world record.

Actually, that is a false assumption.
The "Technical Specification" for IGC Approved FR specifically requires that
any "interference" (such as opening the box or electronically fiddling with
the unit) will activate an internal device that erases the Digital Security
code of the box, which is unique to each FR serial#.
So all traces generated during or after the "interference" will not
validate.
Their security signature will be invalid, as any IGC reading program will
say.
If you try to manually edit that file, it will generate yet another
inconsistency in the signature.

The only way to get a "violated" FR back to "secure" mode is to send it back
to the manufacturer, which has to insert a new, one-time digital security
code that is specific to that particular unit's serial #, assigned by the
IGC.

In other words, if you open your FR, the only way you will get it to
generate a secure (valid) tracklog is by involving the manufacturer and the
IGC.

I don't think this is something you can work around at home, no matter how
much of an electronic genious you are.