I think it would be interesting to try. You've got a decently large area, and not a lot of birds, so your plants may have a fighting chance. I agree seedlings may get demolished, so you'd probably want to have the garden/forest pretty well established before letting the birds loose.
I've never grown grains, so I have no idea how those would do in the type of setting you're proposing.
Strawberry plants, I'm thinking those would take it pretty hard from the birds. Raspberries, boysenberries, blueberries, blackberries should all do well. Grapes should also be able to survive.
I'm not sure the birds would mess with garlic or onions, but they'll still scratch around the plants.
I think asparagus would be decimated as those yummy tender shoots appeared.
What else specifically are you thinking about planting?
I think dense planting may help. I'd also try to set up an area specifically for dust bathing. The birds will still scratch around the plants, but well established plants can handle some scratching. Use a lot of mulch, that will help protect the roots and also increase the insect population for the hens to eat.
If you go forward with this project, I'd love to see how things work out for you!