>This would be wildly slow (if it runs in userspace using FUSE)

NTFS-3G through FUSE is what most people are using. It's slower, but not that slow.

ntfs3 hasn't seen that much large-scale deployment, and you don't have to look very far to find people complaining about ending up with a messed up filesystem from it. I'd put a very modest level of trust in it not eating your data.