Gilad, thanks for your help.
I think I'm going to give up at this point. I can't work out what the analogous Javascript enabling location is on OS X, and the only guidance I can find on the web for that, seems to be years old.
I've utterly failed to build the Adobe's XMP SDK: the XCode project appears to be for a rather old OS X version, and the cmake instructions appear to assume they're being used on a Linux box, and fail on OS X (I'm not a cmake enthusiast, so can't really debug this without taking the rest of the day).
As far as I can see, from the XMP Specification Part3, Sect. 1.6.1, the XMP is in the correct place, so Reader's failure to find it is inexplicable (and provides yet another reason for avoiding this dreadful program).
Since all I'm trying to do is verify I'm doing The Right Thing, rather than actually get the XMP out again, it's not worth wasting any more of my or your time.
So thanks again, Gilad. All the best....