Unfortunately your options are very limited on the Mac. I have used Macs as my primary computers since 1985, and I'm a moderator at MacRumors.com. But when it's time to make maps.... I fire up the Windows PC.

"Back in the day", I used Virtual PC on my Mac to run Windows software, but that left a lot to be desired in terms of performance so I eventually got a dedicated system. With today's faster Macs, that might be less of a problem.
I believe that there are OpenStreetMap tools that are Java-based and work on Macs the same as Windows, but have never spent any time with those.
Making transparent overlays is trivial, it's just a matter of setting a map property and really has nothing to do with the way you make the map. It can easily be done with gmaptool or MapsetToolkit on Windows. But, IIRC, Mapwel also gives you that option.