I've spent about a week trying to get ConTeXt to find the fonts in the Mac's /Library/Fonts directory (and search recursively as well). The ConTeXt support documents are rather fragmented, sometimes out of date and often contradictory. This is not to criticise those behind the project. I think that the number of people working (unpaid and in their own time) on ConTeXt is very small compared to those working on TeX and LaTeX and, at the same time, those are mature technologies with little changing and, therefore, not much demand for updated documentation. Anyway, I finally found a solution on Stack Exchange which did the trick.
The great secret to getting tech to work, even something as apparently simple as getting ConTeXt to recognise the path to the fonts, is persistence. That probably applies to getting anything to work. Whether I'll ever have the patience to see through a complete XML workflow remains to be seen! And then there is the question as to whether it will repay the effort.