Why not plant grass and make the flower bed in another location so you would not be limited to plant selection?
6" of soil is not too bad, and I would bet there are lots of hardy perennials like Black Eyed Susan and daylilly that would grow well.
Or you could always add more soil.

Mike

I have some sedum that I dug up one spring, meant to replant, never did, and it's been growing first on a bare concrete step and now on top of a concrete sidewalk for two years. The dirt washed off the roots a long time ago and apparently the plant doesn't much care.
Based on this I would have to recomend sedum Autumn Joy or S. spectabile for thin soils -in addition to the b.e susan and daylily already mentioned.

A second vote for sedum. I've got some ground cover sedum that seems to survive on air. It would consider six inches of soil to be a luscious treat.

we do have some sedum at other locations, we will have to try some here.
Thanks for the suggestions