See, there I go mixing it up with the steelforce again.
Yeah, you're right. The concave can be a problem. The Z delta has it, but not as extreme so its still workable on the stone.
The file method would probably be your best bet. Just make sure you pull the file down the blade and not across the blade.
You don't push a file across the blade to get the best edge. You pull the file down to the point with the edge of the blade between the (the word escapes me)... Uh, crud, not the top of the teeth, but between the teeth. Shoot, Joey, drive over here and I'll show you what I'm talking about. A fine Mill Bastard file works best.