Hi,
I have looked at some online directories for Cape Bretton and the number of McDonalds is astounding.
This may be clutching at straws a bit, but how about trying to follow up on the "son was a minister" angle. Was the family Presbyterian? There is a book online (Free) called the History of the Presbyterian Church in Cape Breton.
http://www.archive.org/details/thehistoryofthep00murruoft
In the left hand pane, you can click "read online" and it pops up with a search box. There are lots of McDonalds in this book (you will see little balloons on the bar at the bottom of the page), but there are biographical sketches of many of them.
It looks as if it wasn't unusual for the ministers to have obtained degrees from Dalhousie University and from the Presbyterian College (both in Halifax NS). I don't know if either of these institutions have yearbooks online - or class photos/bios.
This may help narrow down options or confirm a family that you think may be right - if you can tie it to a minister??
I know this lead is a HUGE longshot, but it might help --

Good luck - this is a tough one for sure.
RK