I agree with you that creation of an image file, organizing a file system on it and loop-mounting that file system is considered relatively safe - with a catch I shall explain below.

I was reading the ntfs-3g write up, they mentioned this in the Wiki :-

NTFS-3G supports all operations for writing files: files of any size can be created, modified, renamed, moved, or deleted on NTFS partitions. Transparent compression is supported, but there is no support for encryption. Support to modify access control lists and permissions is available.
From what I read, if you want to use NTFS-3g to fix access control list and permission on a Windows system, you might not succeed because of the catchy support of ACL and permission. However, if you want to implement what Knoppix needed, ie compressed loop mount a file on NTFS, it would appear to be safe.

On the other hand ......

I did mention before for Knoppix 6.2.1 ( which I did not check if it is still valid after that ), it is in my humble opinion that knoppix does not shutdown the persistent file system properly, more so for FUSE. If you look at the shutdown script, basically it did not manage to unmount the UNIONFS/aufs2. The FUSE user space driver is terminated before the persistent file system got unmounted. If FUSE keeps buffer, the buffer content is it flushed ? I am not sure. Specifically I noticed when using vanilla Knoppix 6.2.1 with a ext2 persistent store loop mounted on NTFS, I noticed the system had to fsck the persistent store, and each time there is inconsistencies and it requires manual intervention. That was Knoppix 6.2.1 anyway .....