Your description suggests native plants might best be suited to this tough location. I don't know a *thing* about native plants in Massachusetts, but maybe this link will help?

Here is a link that might be useful: Native Plant Resources, Massachusetts

Not much will grow in 6 inches of soil over bedrock except perhaps groundcover sedums.

I've grown bearded iris with less soil than that. My ultimate solution, though, has been a project to remove the bedrock. Fortunately, it is just one small area to deal with.