How does the ISAN affect copyright registration?
It doesn't - because the ISAN is not related in any way to copyright, in either the European or North American sense of that term. The ISAN is an identification number without any legal implication or meaning. It has no value as prima facie evidence regarding the copyright status or ownership of a work.
The scope of the ISAN standard clearly states:
"The issuance of an ISAN shall in no way be related to any process of copyright registration, nor shall the issuance of an ISAN provide evidence of the ownership of rights in a work.