Without knowing where you are or the climate you are in. . .
I would go look around abandoned parking lots or other paved over places. The weeds growing there don't need much soil usually. But they might not like the loose structure of leaf piles over wood and sticks. If they don't take, maybe make compacted
seed balls or if the lot is really abandoned, crack off some
concrete with the plants
roots inside.
If there is a good sized forest near you, try looking for weed seeds out of the deepest piles of old leaves, or on the edge of a tree fall. But I have to say please be respectful of number of plants you need to disturb to get the seeds, it is not the same as taking plants/seeds from an abandoned building lot, IMHO.
Could you plant sprawling plants next to the pile and train them over it? Sorta like layering a plant for propagation, but layering them in place.
Does moss grow in your area? While it wont build soil fast it does give weeds a place to take
root on bare rocks., you could put down chunks and plant some seeds into the moss.
What about putting (peat pot) potted plants into the bed and letting them move out from their starting soil.
If you just left it for a year, I bet there would be something growing over it by the end.