Cynthia, I can't speak from personal experience, but I have seen many steep hillsides in my area that are stablized with hosta and mulch. It seems to work quite well.

Gold Standard on a hillside

Could you put a semi-circle of 4-8" rocks around the down-slope side of each plant? It will slow the water down and help with the stabilization of roots.
Peggy

I have a bunch of hosta on my back hill with mulch. We haven't had a lot of erosion issues, I would say our biggest problem is keeping the mulch in place right at the bottom. But I have not had a problem with the hosta or their roots getting washed out!

Here is a link that might be useful:

Thanks Lois, don, dmid, and peggy hosta! Now that I see your pictures again, don, I remember your fabulous hill and love yours, too, dmid! I think I will dig in some rocks peggy. Good idea. I am so pleased. I can't wait to get the hosta in. Now, what to do with all those little pots of liriope and mungo grass...lol.