Is there any documentation in existence though for when the export bracing option came about? Was it available as early as the Mustang's intro?
Don't recall any but that is not to say there isn't something out there. A couple of places to look might be the first set of cars sent to Allan Mann (100025, 100026, 100027, 100030) since there is possible some period pictures of one of these out there. Not certain that there were built as exports, just a possibility and very very early cars. Possibly the Falcon side also since they received export braces though different ones. Just a couple of possibilities.
One thing I think it is important to clarify is that not all exported cars got the export bracing...lots of T5s did not have it.
Your comment about "lots" of T5s not receiving the package is interesting. Haven't seen that on original T5's in this country or the few I saw in Germany. Have seen my share but of course not all of them. A fair number of export cars were/have been "converted" to T5 by owners in the 80's since that got them more attention in magazines and at shows from what I say and was told by the owners. Don't mean to take the discussion elsewhere the shocks are interesting enough
Export cars are a whole 'nother rabbit hole with lots of one offs sometimes related to almost monthly changes in laws and regulations in other countries. Cars built with strange engines that were identified by numbers rather than letters and so on. But those are not likely connected to this discussion currently
