Signing external data without URL

I'm working with a scheme where data is external to the
signature but does not necessary be available for the
validator by an URL.

To put a single Reference pointing to a Manifest
containing a single Reference seems like a possibility
but an ugly one.

To NOT specify an URL in a Reference (turning of de-referencing)
and maybe add a static "Type" attribute seems possible but is that
recommendable (fully compliant)?

Thanx
Anders Rundgren