I'm sorry but I am not aware of a particular groundcover for such a location that is readily available from seed. I would consider a native meadow grass mix if that would not get too large for your purposes. Or perhaps go with on of the vining groundcover planted as wide spacings to be allowed to fill in over time.
Perhaps your local county Extension Office can provide more ideas or options:
750 South Perry Street, Suite 400
Lawrenceville, GA 30045-4804
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