What a pain. So it turns out that Microsoft NTBACKUP uses Unicode UTF-16LE NO BOM for creating and reading a .BKS file.

So far I've not been able to get VBScript with FSO to create one. If anyone else has had any luck with this please let me know and share the code.

I can use UltraEdit to save the file as Unicode UTF-16LE NO BOM and NTBACKUP will use it, but I'd really like to finish this off with an automated script if possible. Otherwise I'll have to build the BKS file manually for everyone that I want to set this backup for.