Eureka! The gimgunlock tool for Mac may be old, but it appears to have done the trick with the locked v.4 .img file.
For other Mac users out there, the process is just to decompress and merge (automatically) the various .rar files into the single .img file and then follow the instructions for using gimgunlock
1) open Terminal
2) drag the gimgunlock icon into Terminal
3) drag the .img file into Terminal and
4) run it.
This appears to overwrite the original locked .img file.
All one has to do then is move the unlocked .img file to the Garmin file (on the microSD card) in the Garmin GPS. Except...
There was a little technical hitch after I disconnected the Garmin from the Mac. When I turned the Garmin GPSMap 62s back on, it did not boot all the way, but rather got stuck on "loading profiles".
I turned the GPS off, removed the microSD card, turned the GPS back on, which then booted up all the way OK, turned the GPS off again, reinserted the microSD card, turned it on again and lo and behold the unlocked .img loaded perfectly. The same map is also perfectly accessible through Garmin Basecamp on the Mac.
Having said all the above, I am astonished at how little hiking trail information there is on v.4. I have seen some comments in other places as to the superiority of v.2 and v.3 over v.4 for mountain biking trails, and wonder if I would be better off with an earlier version. But this may be the wrong place and or post in which to ask that question.