We don't really get hail here, and when we do it's itty bitty. Biggest I've ever seen was maybe the size of my pinky tip, happened in the winter, and I could crumble it relatively easily. And, it only happened once in my life. We get hail maybe twice a year, in the winter. So, I really have no
experience with
gardening with the stuff.
But, I do recall reading in
The Farmer's Handbooka lot of recommendations for how to deal with hail (as these
books were written for the Himalayan area, and so hail and monsoons were something very regular and to be prepared for). When setting up a nursery for starting plants, they would put a thatch roof over it, to protect it. They also utilized tree cover (like you were mentioning) and thatch over their baby trees, too, to protect from hail. Here's a direct link to the specific chapter in The Farmer's Handbook #3:
http://www.permaculturenews.org/resources_files/farmers_handbook/volume_3/6_home_nursery.pdf
There might very well be more ideas in the rest of the books, but this was the one that stuck out most to me (mostly because I couldn't really fathom having to
shelter from rain/hail, as powerful precipitation just isn't something we get here!).
Since the books are in PDF format (and FREE!), you could look at them here:
http://www.permaculturenews.org/resources_files/farmers_handbook/ or download them here:
http://www.green-shopping.co.uk/ebooks/free-ebooks.html. Once you have them open, you can do a ctrl+f and search each book for "hail" and see if any other suggestions come up!
Oh! Just search through book 2 of the Farmer's Handbook, and they recommend thatching over the plants that have been selected for seed saving, so as to offter them extra protection. (Page nine of chapter 8 of seed saving:
http://www.permaculturenews.org/resources_files/farmers_handbook/volume_2/8_seed_saving.pdf)
They also suggest using green manures (cover crops) to protect the soil from heavy rains, and I would think it would help with hail, too (Book four, chapter 2 Green Manures:
http://www.permaculturenews.org/resources_files/farmers_handbook/volume_4/2_green_manures.pdf).
I hope that helps!