Lets say I have a site called: Customer1 Which represents the documents for that customer.
Inside the Customer1 site, I have web properties(property bag) like: ClientCode: 123 CustomerName: Customer1 YearAcquired: 2012
Lets say we have 1 content type called deliverables, this content type has a field called ProjectName.
All our sites point to an enterprise search center.
Now the user wants to make searches like this: ContentType=Deliverable and ClientCode=123 and ProjectName:xxx
As you can see the content type deliverable does not have the client code, the client code is in a web property of the web where the document belongs to and I am combining managed properties from the document and web properties too.
I doubt this is possible the way it was designed (not by me)