@Atari800XL should know: https://forums.mydigitallife.net/threads/uup-dump.75052/page-22#post-1369572
The --ref= switch is in documentation. I have been creating delta wims (that look similar to UUP's) using wimlib-imagex before UUP's existed and I have been using this switch to apply them.
According to WSUS, it seems the missing FOD files links are not ment or exist for public WU release only the UUP platform allowed us to have them, like it provide many unofficial files (i.e. EditionSpecific-EnterpriseG-Package) officially, softreboot is intended for server images openssh/printing is for x64 only zh-hk packs always released delayed bg-bg, he-il, hr-hr, sl-si never got texttospeech in previous versions