So it seems I can no longer have my projects set up as shown in my original post - it seems rule files must now be in the same project as any file they reference. So I have two choices:
1] I copy my library of facade images and my rule file into every project --- nope.
2] I move my aerial photos and DTMs, which are specific to each project, into my 'Libraries' folder --- ok, but seems illogical captain.
Not a big problem for me to adapt to this, it means I'll no longer have wierd cross-references in my rule file so they'll be a bit easier to understand......but why can't we be free?