I'm not an organic chemist, but those warnings might be for the crystalline form of ascorbic acid, and not for when it's dissolved in something tasty like apfelwein.
I don't imagine that fermentation would destroy it-- just my instinct, though.
On Deck: Jamil's Vanilla Robust Porter
Fermenting: Orange Blossom Mead
Kegs: Element 56 Pale Ale, Ron's Belgian Blonde, Summer'n Saison, Furloughktoberfest '09, Grateful Pale Ale, Sam Adams Cream Stout Clone, EdWort's Apfelwein