Link Hashkeys should be generated using the Business Keys participating in the Link, and not the Hashkeys of of the Hubs. Hashing a hash is just bad design.
If your LINKs have Dependent Childs, for e.g. Order Line Item as show below, it is crucial that the Order Line Item is included the LINK Hashkey.If the Dependent Child is not included the generation of the LINK Hashkey, the JOINs to the LINK table will not be easy.
Why would you do that? HashKeys and HashDiffs are binary generated using a hashing algorithm like MD5 or SHA-1. Just store them as binary and effectively halve your storage and double your I/O! No need to convert them to the Char to store them as Varchar.