I looked at American Cranberries though as a cool way to grow food and ground cover at the same time. I have wet clay soil on a slope so I think if I could get them to mat I could say good be to mowing that part of the lawn.

As far as the granite ledge causing problems I doubt it. When I lived in Switzerland we hiked way up in the mountains in August and there were actually families with pack mules and berry rakes picking wild blueberries and cranberries.

It you have very rocky conditions I recommend planting seeds or starting very small cuttings early in spring so that the root systems can develop into the natural dirt crevices of where you want them planted.

I will tell you I have wild low bush blueberry and it's suitable as understory in the woods or as sparse vegetation in a rocky area but not as ground cover in a yard maybe edging the sides of a long dirt driveway. I think cranberry though would be good ground cover if conditions were suitable for it to mat, a bit like periwinkle.

Have you considered native seeds from B & T?

http://b-and-t-world-seeds.com/alaCarth.asp?searchFor=vaccinium

P.S. I don't own or am I affiliated with B & T although I did order Touch Me Not seeds from them for my mom once. They keep re-seeding.